PSEB 7th Class Science Solutions Chapter 15 Light

PSEB Solutions for Class 7 Science Chapter 15 Light

Science Guide for Class 7 PSEB Light Intext Questions and Answers

Think and Answer (Textbook Page No. 186)

Question 1.
The process of change in direction of light after falling on a mirror is called ……………….. of light.
Answer:
Reflection.

Question 2.
Angle of incidence is equal to angle of reflection. (True/False)
Answer:
True.

PSEB 7th Class Science Solutions Chapter 15 Light

Think and Answer (Textbook Page No. 189)

Question 1.
The distance between the object and the mirror is ……………… to the distance
between the image and the mirror.
Answer:
Equal.

Question 2.
Thes plane mirror has to be placed vertically on the graph paper. (True/False)
Answer:
True.

Question 3.
The plane mirror should be vertically upright.
Answer:
Right.

Think and Answer (Textbook Page No. 191)

Question 1.
The image of the sun formed by the concave mirror on the wall is …………………… image.
(Real/Virtual)
Answer:
Real.

Question 2.
Is the image of a candle seen using a concave mirror same as that the image of the Sun seen on the wall ?
Answer:
In both cases the rays of light are reflected from a concave mirror to form a real image.

Think and Answer (Textbook Page No. 192)

Question 1.
When the object is very far from the concave mirror, the image formed is ………………….
and
Answer:
Real, inverted/small in size.

Question 2.
The image formed on the screen is real.
Answer:
True. (True/False)

Question 3.
For a concave mirror, the image formed is always real.
Answer:
False.

PSEB 7th Class Science Solutions Chapter 15 Light

Think and Answer (Textbook Page No. 195) (True/False)

Question 1.
The bright spot of light obtained on the paper is the image formed due to effect.
Answer:
Converging.

Question 2.
The image obtained on the paper is the virtual image.
Answer:
False.

Think and Answer (Textbook Page No. 196) 

Question 1.
The convex lens is placed between the object and the screen. (True/False)
Answer:
True.

Question 2.
For a convex lens, the image formed is always real. (True/False)
Answer:
False.

Think and Answer (Textbook Page No. 198)

Question 1.
White light is composed of ………………. Colours. (True/False)
Answer:
Seven.

Question 2.
When disc with seven colours is rotated, it appears to be red. (True/False)
Answer:
False.

Question 3.
Name the seven colours of which the white light is made of.
Answer:
The seven colours of white light are:

  1. Violets,
  2. Indigo,
  3. Blue,
  4. Green,
  5. Yellow,
  6. Orange
  7. Red.

Collectively called VIBGYOR.

PSEB 7th Class Science Solutions Chapter 15 Light

PSEB 7th Class Science Guide Light Textbook Questions and Answers

1. Fill in the Blanks:

(i) An image formed by a ………………….. mirror is always of the same size as that of object.
Answer:
plane

(ii) In a plane mirror, the left hand of a person appears to be the ………………… hand in the image and ……………. hand appears to be the left hand in the image.
Answer:
right, right

(iii) The image formed by a convex mirror is always ………………… and ……………….. in size.
Answer:
erect, smaller

(iv) Convex lenses are ………………… in the middle and concave lenses are ……………….. in the middle than at the edges.
Answer:
thin, thick

(v) A prism splits the white light into ……………….. colours.
Answer:
seven

2. State True or False:

(i) There is reflection of light through a lens.
Answer:
False

(ii) The ray of light coming towards the plane mirror is called the reflected ray.
Answer:
False

PSEB 7th Class Science Solutions Chapter 15 Light

(iii) The image formed by a plane mirror is always in front of the mirror.
Answer:
False

(iv) A concave mirror is a part of hollow sphere of glass, whose outer side is coated with silver layer and reflection takes place from the inside.
Answer:
True

(v) Concave lens always forms a virtual, erect and smaller image of the object.
Answer:
True

3. Choose the Correct Answer:

Question (i)
Which of the following does not show reflection of light ?
(a) Plane mirror
(b) Concave mirror
(c) Convex mirror
(d) Card board.
Answer:
(d) Card board.

Question (ii)
Which is used for rear view in cars and other vehicles:
(a) Concave mirror
(b) Convex mirror
(c) Convex lens
(d) Concave lens.
Answer:
(b) Convex mirror.

Question (iii)
The image of an object formed by a concave lens is always ?
(a) Real and diminished
(b) Virtual and larger
(c) Real and larger
(d) Virtual and diminished.
Answer:
(d) Virtual and diminished.

Question (iv)
The process of splitting of white light in seven colours on passing through a prism is called:
(a) Reflection of light
(b) Refraction of light
(c) Bending of light
(d) Dispersion of light.
Answer:
(d) Dispersion of light.

PSEB 7th Class Science Solutions Chapter 15 Light

4. Match the Column ‘I’ with Column ‘II’:

Column ‘I’ Column ‘II’
1. Mirror used by dentists (a) Spectacles
2. Rear view Mirror (b) Microscope
3. Magnifying glass (c) Vehicles
4. Concave Lens (d) Concave

Answer:

Column ‘I’ Column ‘II’
1. Mirror used by dentists (d) Concave
2. Rear view Mirror (c) Vehicles
3. Magnifying glass (b) Microscope
4. Concave Lens (a) Spectacles.

5. Very Short Answer Type Questions:

Question (i)
Is the image formed by a plane mirror real or virtual ?
Answer:
The image formed by the plane mirror is Virtual.

Question (ii)
Which lens forms a real image of an object ?
Answer:
Convex lens forms a real image of the object.

Question (iii)
Which optical instrument uses convex lens ?
Answer:
Convex lens is used in a microscope.

Question (iv)
What is the seven rainbow coloured disc called ?
Answer:
The seven-rainbow coloured disc is called Newton’s disc.

6. Short Answer Type Questions:

Question (i)
Differentiate between Real and Virtual images. Give examples.
Answer:
Difference between Real image and Virtual image:

Real image Virtual image
1. The rays of light coming from the object are actually found to meet at a point after reflection or refraction. 1. The light rays coming from an object do not actually meet but appear to meet at a single point after reflection or refraction.
2. They are always inverted. 2. They are always erect.
3. Real image can be obtained on a screen.

Example : (a) If the object is too far away from the concave mirror, the image becomes too small and inverted. When the object is brought closer to the concave miiror, the image after reflection becomes large and real.

(b) If the object is in front of a convex lens then the rays coming from that will form a real, inverted image after refraction.
PSEB 7th Class Science Solutions Chapter 15 Light 1

3. Virtual image can not be obtained on a screen

Example : (a) If the object is between the focal point of the concave mirror and the object, then the image is large, upright and virtual.In the convex mirror for each position of the object, the image obtained is virtual, erect and small.

(b) If the object is between the focal point of the convex lens and the lens, then image is virtual, erect and large.
PSEB 7th Class Science Solutions Chapter 15 Light 2

PSEB 7th Class Science Solutions Chapter 15 Light

Question (ii)
A person is standing 5m away from a plane mirror. How far will his image be:
(a) from the mirror
(b) from the person himself ?
Solution:
We know that the image formed in a plane mirror is at the same distance behind the mirror is as the object is in front of the mirror. So,
(a) Distance of image from the mirror = Distance of person from the mirror
= 2 meters

(b) Distance of the image from the person = Distance of the person from the plane mirror + Image distance from the mirror. = 2 meters + 2 meters
∴ Distance of the image from the person = 4 meters

Question (iii)
Give two uses of concave mirrors.
Answer:
Uses of concave mirrors:

  • Concave mirror is used as a having mirror, as the concave mirror produces a large and erect image when the mirror is placed close to the face.
  • Concave mirrors are mostly used on the headlights of vehicles. Here the bulb (light source) is placed on the principal focus of the concave mirror.

Question (iv)
State two differences between convex lens and concave lens.
Answer:
Difference between convex lens and concave lens:

Convex lens Concave lens
(1) It is thick in the middle and thin at the edges. (1) It is thin in the middle and thick at the edges.
(2) It generally forms a large image of the object. (2) It forms a small and virtual image of the object.

Question (v)
Of how many colours is white light made of ? Name them.
Answer:
White light consists of seven colours. Their names are:

  1. Violet,
  2. Purple (Indigo),
  3. Blue,
  4. Green.
  5. Yellow,
  6. Orange and
  7. Red.

Question (vi)
Ravi is observing his image in a plane mirror. The distance between the mirror and his image is 6 m. If he moves 2 m towards the mirror, then the distance between Ravi and his image will be:
Answer:
Given, the distance between the plane mirror and the image = 6 meters
We know, the distance between the object (Ravi) and the mirror = Distance between the mirror and the image
Distance between Ravi and mirror = 6 meters (Given.)
PSEB 7th Class Science Solutions Chapter 15 Light 3
When he moves 2 meters forward then the distance between Ravi and mirror will be
= 6 meters – 2 meters
= 4 meters.
Now,
Distance between Ravi and his image = Distance between Ravi and Plane mirror + Distance between plane mirror and image
= 4 meters + 4 meters
= 8 meters

7. Long Answer Type Questions:

Question (i)
State and explain the characteristics of the image formed by a plane mirror.
Answer:
Characteristics of the image formed by the plane mirror:

  • The image of the burning candle (object) is seen behind the mirror at the same distance. Therefore the image formed by a plane mirror is always formed as far behind the mirror as the objects is in front of it.
  • Since the image of the candle behind the mirror cannot be obtained on the screen, therefore, The image formed by the mirror is always virtual.
  • When we place the candle upright the image formed is also upwards. This shows that the image formed by a plane mirror is erect.
  • When we compare the size of the candle and its image formed in a plane mirror, they are of the same size. This shows the size of the object and the size image formed by the plane mirror is equal.
  • The distance between the candle and the mirror is equal to the distance between the image of the candle and the minor.
  • The image formed by a plane mirror is laterally inverted. That is, the left side of the object appears to the right side of the image and the right side of the object is to the left side of the image.

Question (ii)
What is meant by dispersion of the light ? Explain by using a Prism. Which natural phenomenon is associated with dispersion of light ?
Answer:
PSEB 7th Class Science Solutions Chapter 15 Light 4
Dispersion of Light.
It is the phenomena of spliting the white light into its seven colours. The process of its separation into seven colors is characterized by a white ray of light passing through the prism as shown in the figure.

Place a white screen on the other side of prism. You will see that the white light will be splited in seven colors. These colors form a band of the seven colors received on the screen called the spectrum. Due to the different wavelengths of the seven colors, these seven colors diverge at different angles. These seven colors are: (1) Violet, (2) Indigo, (3) Blue, (4) Green, (5) Yellow, (6) Orange, (7) Red.

We can remember these names with the word VIBGYOR formed from their first letters. You may have seen rainbows forming in the sky after rain. It causes the rays of the sun’s white light to be dispersed from the water droplets hanging in the atmosphere. Here the water droplets act like a prism and split up the white sunlight into its seven colours.

PSEB 7th Class Science Solutions Chapter 15 Light

PSEB Solutions for Class 7 Science Light Important Questions and Answers

1. Fill in the Blanks:

(i) An image that cannot be obtained on a screen is called …………………..
Answer:
virtual image

(ii) Image formed by a convex …………………. is always virtual and smaller in size.
Answer:
mirror

(iii) An image formed by a ………………….. mirror is always of same size as that of an object.
Answer:
plane

(iv) An image which can be obtained on a screen is called a ………………. image
Answer:
real

(v) An image formed by a …………………. lens cannot be obtained on a screen.
Answer:
concave

2. Match the Column ‘A’ with Column ‘B’:

Column ‘A’ Column ‘B’
(i) Convex lens (a) Rainbow
(ii) Real, inverted and equal image (b) Concave mirror
(iii) Alphabetical process (c) Magnifying glass
(iv) Virtual image (d) Convex lens
(v) The reflecting surface is inward (e) Cannot be obtained on screen

Answer:

Column ‘A’ Column ‘B’
(i) Convex lens (c) Magnifying glass
(ii) Real, inverted and equal image (d) Convex lens
(iii) Alphabetical process (a) Rainbow
(iv) Virtual image (e) Cannot be obtained on screen
(v) The reflecting surface is inward (b) Concave mirror.

3. Choose the Correct Answer:

Question (i)
Light travels in:
(a) Straight lines
(b) Curved lines
(c) Circles
(d) None of the above.
Answer:
(a) Straight lines.

PSEB 7th Class Science Solutions Chapter 15 Light

Question (ii)
Erect, virtual and diminished image is formed in:
(a) plane mirror
(b) concave mirror convexo.
(c) convex mirror
(d) concave mirror
Answer:
(c) convex mirror.

Question (iii)
To see an object it is necessary:
(a) Source of light
(b) Source of light and an object
(c) Source of light, object and an eye
(d) None of these.
Answer:
(c) Source of light, object and an eye.

Question (iv)
After passing through a prism, light is split up into:
(a) two colours
(b) five colours
(c) seven colours
(d) six colours.
Answer:
(c) seven colours.

Question (v)
To see magnified view of teeth, dentists use:
(a) plane mirror
(b) concave mirror
(c) convex mirror and convex mirrors.
(d) combination of concave
Answer:
(b) concave mirror

Question (vi)
Which mirror is used to get magnified and real image ?
(a) convex mirror
(b) plane mirror
(c) concave mirror
(d) none of the above.
Answer:
(c) concave mirror.

Question (vii)
Convex lens is:
(a) thick at the centre and thin at the edges
(b) thick at the edges and thin at the centre
(c) uniformly thick
(d) irregularly thick.
Answer:
(a) thick at the centre and thin at the edges.

Question (viii)
When Newton disc is rotated fast which colour is seen ?
PSEB 7th Class Science Solutions Chapter 15 Light 5
(a) Black
(b) White
(c) Blue
(d) Yellow.
Answer:
(b) White.

PSEB 7th Class Science Solutions Chapter 15 Light

Very Short Answer Type Questions

Question 1.
Give one property of light.
Answer:
Light travels in a straight line.

Question 2.
Which surface acts as a mirror ?
Answer:
Any smooth and polished surface.

Question 3.
If you are standing in front of a plane mirror and observing your own image what is the relation
Answer:
Distance of the object from the mirror = Distance of the Image from the mirror.

Question 4.
What are spherical mirrors ?
Answer:
Spherical Mirrors. Spherical mirrors are the part of hollow spherical reflecting surface of which the mirror is a part. The spherical mirrors are of two types:

  1. Concave mirrors
  2. Convex mirrors.

Question 5.
One wants to get an enlarged image of an object in a mirror. What type of mirror should one use ?
Answer:
Concave mirror.

Question 6.
What type of mirror is used in automobiles for the benefit of drivers ?
Answer:
Convex mirror.

Question 7.
What type of mirrors are used in search-lights and head lights of cars ?
Answer:
Concave mirror.

PSEB 7th Class Science Solutions Chapter 15 Light

Question 8.
Name the mirror in which the image of object is always diminished and virtual.
Answer:
Convex mirror.

Question 9.
Name the kind of mirror which produces a diminished real image.
Answer:
Concave mirror.

Question 10.
Write a note on convex lens.
Answer:
PSEB 7th Class Science Solutions Chapter 15 Light 6
Convex lens.
A convex lens is thick in the centre and thin at the edges. This lens has the capability to converge a beam of light. So this lens is also called a converging lens.

Question 11.
What is a concave lens ?
Answer:
PSEB 7th Class Science Solutions Chapter 15 Light 7
Concave lens.
A concave lens is thin at the centre and thick at the edges. This lens has a capacity to diverge a beam of light. So this lens is also called a diverging lens.

Question 12.
Name the seven constituent colors of light.
Answer:
Violet, Indigo, Blue, Green, Yellow, Orange, Red.

Question 13.
Describe a natural phenomenon in which seven colors of light are displayed.
Answer:
View of the rainbow after rains.

Short Answer Type Qucstions

Question 1.
Why should not we see towards sun through a convex lens ?
Answer:
A convex lens is a converging lens. It converges a beam of parallel rays coming from the Sun at the focus. If a piece of paper or cloth is placed at the focus of a convex lens from the sun, these catch fire. So, if we see towards the sun through a convex lens, large amount of heat will be produced which is very harmful for the eyes. Hence we should not see the sun through a convex lens.

Question 2.
What is a virtual image ? Give one situation where a virtual image is formed.
Answer:
Virtual image. An image which cannot be obtained on a screen, is called a virtual image.
Image formed in a plane mirror is always virtual.
Image in a concave mirror is virtual when object is placed very close to the mirror.
Image formed in a convex mirror is always virtual.

PSEB 7th Class Science Solutions Chapter 15 Light

Question 3.
State two differences between a convex and a concave lens.
Answer:
Differences between a Convex and a Concave lens:

Convex lens Concave lens
1. It is thick at the centre and thin at the edges.

PSEB 7th Class Science Solutions Chapter 15 Light 8

It is thin at the centre and thick at the edges

PSEB 7th Class Science Solutions Chapter 15 Light 9

2. The image is generally formed on the other side of the lens (except when the object is between F and optical centre). The image is formed on the same side of the object.
3. It converges a beam of light to a It diverges a beam of light as coming from a point.

Question 4.
Give one use each of a concave and a convex mirror.
Answer:
Use of concave mirror. It is used as reflector in a searchlight or car head-light.
Use of convex mirror. It is used in cars and other vehicles as a rear view mirror.

Question 5.
Which type of mirror can form a real image ?
Answer:
A concave mirror forms a real image.

Question 6.
Which type of lens forms always a virtual image ?
Answer:
Concave lens.

Question 7.
Give the conditions necessary for seeing an object.
Answer:
Conditions for seeing an object. To see an object the following three conditions are required to be satisfied:

  1. Source of light to make the object visible.
  2. The object.
  3. Eye.

Question 8.
What are the uses of mirrors ?
Answer:
Uses of Mirrors. We use mirrors in our daily life for one or the other purpose.
(i) Plane Mirror. Plane mirrors are used in the barber’s shops and in homes as looking glass. It is also used in periscope and other optical instruments.

(ii) Concave mirror.

  1. It is used as reflector in a search light or in a car head light.
  2. It is used in the solar cookers to converge the radiations coming from the sun.

(iii) Convex mirror.

  1. It is used in the cars and other vehicles as a rear view mirror to have look at the vehicles coming behind.
  2. It is used in amusement parks for showing strange looking images of people who stand in front of them.

Long Answer Type Question

Question 1.
Give an experiment to show that white light is made up of seven colours.
Answer:
PSEB 7th Class Science Solutions Chapter 15 Light 10
White light is a mixture of seven colours. To prove that white light is a mixture of seven colours, following activity is performed.

Make a small top with rotating disc. Paint the upper surface of disc with sectorial area proportionately to the seven colours of rainbow. Now rotate the disc when a white colour will appear on the disc.
Light helps us to see the objects around us.

PSEB 7th Class Science Solutions Chapter 15 Light

Light PSEB 7th Class Science Notes

  • A ray coming from source of light falls on the object and after reflection enters our eyes so that we see the objects.
  • Light always travels in a straight line.
  • The reflection from the surface of the object must be the same to see the image.
  • After hitting a surface, the light returns to the same medium in a certain direction. The process of coming back of light is called reflection of light.
  • The ray of light that strikes an object is called an incidental ray and the ray of light which after hitting an object returns to the same medium a particular direction in, is called a reflected ray.
  • The angle between the incident ray and the perpendicular drawn at the point of incidence is called the Angle of Incidence.
  • The angle between the reflected ray and the perpendicular drawn at the point of reflection is called angle of reflection.
  • The angle of incidence and the angle of reflection are always equal. This is called the law of reflection.
  • The image formed on screen is called Real image.
  • The image that cannot be obtained on the screen even if the reflected rays appear to meet
    is called Virtual image.
  • The image formed by a plane mirror is always formed behind the mirror. This image is virtual is erect and of the same size as that of the object.
  • The image formed by a plane mirror is at the same distance behind the mirror as the object is placed in front of the mirror.
  • The image formed by a plane mirror is laterally inverted i.e. the left side of the object becomes to the right of the image and the right side of the object is seen to be left of the image.
  • A concave mirror is a spherical mirror with a reflecting surface facing inwards.
  • A convex mirror is a spherical mirror with a reflecting surface facing outwards.
  • The rays of light coming from a distant object are considered to be parallel to each other and at the point at which they actually meet after being reflected from the mirror or appear to meet, is called the focal point of the mirror.
  • For a concave mirror, a virtual, erect and large image is formed only when the object is placed between the principal focus and the mirror.
  • For a convex mirror the image formed is erect, virtual and smaller in size than the size of the object.
  • Lens is a piece of transparent medium surrounded by two surfaces. The two main types of lenses are – (1) Convex lens and (2) Concave lense.
  • The convex lens is thicker in the middle and thinr at the edges.
  • The concave lens is thin in the middle than at the edges.
  • Convex lens is also called Convergent lens whereas Concave lens is called Divergent lens.
  • The convex lens can be used to see small objects. Hence it is also called magnifying glass or reading glass.
  • On allowing white light to pass through a prism, it splits up into seven colours, which means white light is composed of seven colours.
  • The point at which the rays of light coming from a distant object (such as the sun) meet or appear to meet after suffereing refraction from the lens is called the focal point of the lens.
  • When ray of light enters from one medium to another transparent medium, it changes its path. The prenomenon of light changing its path is called refraction of light.
  • Reflection of light: When light travelling in a straight line strikes the surface a mirror or a polished opaque surface, it changes its direction and returns to the same medium. The process of coming back of light is called reflection of light.
  • Incidental ray: A ray of light falling on the smooth and polished surface is called incident ray.
  • Reflected Ray: A ray of light after striking a mirror changes its direction and returns to the same medium, is called a reflected ray.
  • Angle of incidence: The angle between the incident ray and the perpendicular drawn at the point of incidence is called angle of incidence.
  • Angle of reflection: The angle formed between reflected ray and perpendicular drawn at the point of incidence is called angle of reflection.
  • Point of incidence: The point at which the incident ray strikes the surface of the mirror, is called the point of incidence.
  • Perpendicular: The normal or perpendicular line drawn from point of incidence.
  • Real Image: When the light coming from an object after reflection meet at a point forming an image then such image is called a real image.
  • Virtual reflection: When the rays of light only appear to meet at a point after reflection from a mirror then that point is called virtual image. Virtual image cannot be obtained on the screen.
  • Spherical Mirror: A mirror whose reflecting surface is a part of a hollow spherical surface.
  • Concave mirror: If the inner surface of spherical mirror is reflecting, it is called concave mirror.
  • Convex mirror: If the outer surface of the spherical mirror is reflecting, it is called convex mirror.
  • Light Reflection: The bouncing back of light after when light strikes a smooth surface.
  • Refraction of Light: When a ray of light enter from one transparent medium to another transparent medium, the ray of light change its path. The phenomenon of change of path by light is called refraction.
  • Convex lens: This is a piece of transparent shell that is thicker in the middle than the edges. It is also called converging lens.
  • Concave lens: This is a piece of transparent medium that is thin in the middle and thick at the edges. This lens is also called diverging lens.
  • Focus: The point on the principal axis where the parallel rays of light meet or appear to meet after passing through the lens is called the focus.
  • Focal length: The distance between the principal focus and the center of the lens is called the focal length of the lens.
  • Splitting of light: The process of splitting by when white light passes through a transparent medium (such as a prism of glass) of white light into seven colours is called the splitting of light.
  • Spectrum: It is a band of seven colours obtained after passing white light through the prism. These colors from top to bottom are Violet, Indigo, Blue, Green, Yellow, Orange and Red.

Punjab State Board PSEB 7th Class Science Book Solutions Chapter 15 Light Textbook Exercise Questions and Answers.

PSEB 8th Class Science Solutions Chapter 16 Light

PSEB Solutions for Class 8 Science Chapter 16 Light

PSEB 8th Class Science Guide Light Textbook Questions and Answers

Exercises

Question 1.
Suppose you are in a dark room. Can you see objects in the room ? Can you see objects outside the room ? Explain.
Answer:
No, we cannot see anything in dark. As no light is falling on the objects in the room and they are not emitting any light on their own. So, nothing is visible inside the dark room.
Objects outside the room can be seen, if either there is light outside the room or objects are emitting their own light.

Question 2.
Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection ?
Answer:
Differences between regular reflection and diffused reflection:

Regular reflection Diffused reflection
1. It occurs on a smooth, plane surface. 1. It occurs on a rough, irregular surface.
2. Reflected rays are parallel to one another. 2. Reflected rays are unparallel to each other.

Diffused reflection is not failure of laws of reflection. It is only due to irregularities on the reflecting surface.

PSEB 8th Class Science Solutions Chapter 16 Light

Question 3.
Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
[a] Polished wooden table
[b] Chalk powder
[c] Cardboard surface
[d] Marble floor with water spread over it.
[e] Mirror
[f] Piece of paper.
Answer:
(a) Polished wooden table. Regular reflection, as wooden table has smooth polished surface.
(b) Chalk powder. Diffused reflection due to rough surface of chalk powder.
(c) Cardboard surface. Diffused reflection because cardboard has small irregularities on its surface.
(d) Marble floor with water spread over it. Regular reflection as water gives a smooth surface.
(e) Mirror. Regular reflection due to smooth surface.
(f) Piece of paper. Regular reflection if paper is fine and diffused reflection if paper is coarse.

Question 4.
State the laws of reflection.
Answer:
Laws of reflection.

  1. Angle of incidence (∠i) = Angle of reflection ( ∠r).
  2. Incident ray, reflected ray and the normal at the point of incidence all lie in the same plane.

Question 5.
Describe an activity to show that the incident ray, the reflected ray, normal, the point of incidence lie in the same plane.
Experiment.
Take a white sheet, spread it on a table. Draw a line MM on it. Place a plane mirror vertically on this line. Now throw light on a comb in this ways that a parallel light rays fall on the mirror. Adjust it in such a way that a beautiful pattern of incident and reflected rays is formed. Now mark points A, B, C on incident ray and points D, E, F on its corresponding reflected ray. Switch off the torch and remove the mirror. Join the points and extend line to mirror. ABC will meet MM at O and DEF will all also be meeting at O. OA is incident ray and OF is reflected ray. Draw ON ⊥ MM .
PSEB 8th Class Science Solutions Chapter 16 Light 1
Measure the angle of incidence ∠AON and the angle of reflection ∠FON which would found to be equal. Incident ray, reflected ray and normal, all lie in one plane of paper.

Question 6.
Fill in the blanks in the following.
(a) A person 1 m in front of a mirror seems to be ……………….. m away from his image.
(b) If you touch your ………………….. ear with right hand in front of a plane mirror it will be seen that your right ear is touched with ……………… .
(c) The size of pupil becomes …………………. when you in dim light.
(d) Night birds have ………………. cones than rods in their eyes.
Answer:
(a) 2m
(b) Right, Left
(c) large
(d) more.

Choose the correct option in the Questions 7-8.

Question 7.
Angle of incidence is equal to angle of reflection
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never.
Answer:
(a) Always.

PSEB 8th Class Science Solutions Chapter 16 Light

Question 8.
Image formed by a plane mirror is
(a) virtual, behind the mirror and enlarged.
(b) virtual, behind the mirror and of same size as the object.
(c) real at the surface of mirror and enlarged.
(d) real, behind the mirror and of same size as the object.
Answer:
(b) Virtual, behind the mirror and of same size as the object.

Question 9.
Describe the construction of a Kaleiodeoscope.
Answer:
PSEB 8th Class Science Solutions Chapter 16 Light 2
Construction of a Kaleiodeoscope. It is a toy to form many images by multiple reflections. A Kaleiodeoscope is made up of three strips of plane mirrors inclined at angles of 60° enclosed in a tube. One end of tube has a ground glass plate with a clear glass plate on its inner side. A metallic ring separates the two plates and the space is filled with coloured pieces of glass or broken bangles. A cardboard with a hole in the centre is fixed on the other end of the tube.

Question 10.
Draw a labelled sketch of the human eye.
Answer:
Labelled diagram to show different parts of human eye.
PSEB 8th Class Science Solutions Chapter 16 Light 3

Question 11.
Gurmit wanted to perform activity 16.8 using a laser torch. Her teacher advised her not do so. Can you explain the basis of the teacher’s advice ?
Answer:
Laser torch has very sharp beams of light which can destroy the pupil or retina of the eye.

Question 12.
Explain how can you take care of your eyes.
Answer:
Care of eyes.
Eyes are the most precious gifts of nature. So we should take full care of our eyes by taking atleast following precautions.

  1. We should wash our eyes daily with clean water.
  2. We should not read or work in very bright or dim light.
  3. We should not read in a running vehicle.
  4. We should not rub our eyes.
  5. We should use sunglasses on hot summer day.
  6. We should not look directly at the sun. Also we should not look at the sun during solar eclipse.
  7. We should eat vitamin A rich food for healthy, clear eyes.

Question 13.
What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray ?
Solution:
PSEB 8th Class Science Solutions Chapter 16 Light 4
If ∠i = angle of incidence
∠r = angle of reflection
∠i + ∠r = 90 (given)
But ∠i = ∠r
(According to laws of reflection)
∴ ∠i + ∠i = 90°
2 ∠i = 90
∠r = ∠i – 45°

PSEB 8th Class Science Solutions Chapter 16 Light

Question 14.
How many images of a candle will be formed if it placed between two parallel mirrors separated by 40 cm ?
Answer:
PSEB 8th Class Science Solutions Chapter 16 Light 5
When mirrors are separated by 40 cm and are parallel to each other so that the angle between the mirrors is 0° which is not sulj-multiple of 360°. Then theoretically, the number of images formed would be infinite, but as a considerable amount of light is lost due to reflections so only a limited number of images are seen which is shown in figure.

Question 15.
Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown is figure. Draw the reflected ray from the second mirror.
PSEB 8th Class Science Solutions Chapter 16 Light 6
Solution:
PSEB 8th Class Science Solutions Chapter 16 Light 7

Question 16.
Bhoojo stands at A just on the side of a plane mirror as shown in figure. Can he see himself in the mirror ? Also can he see image of objects situated at P, Q and R ?
PSEB 8th Class Science Solutions Chapter 16 Light 8
Solution:
Bhoojo cannot see his image in the mirror as he is standing outside the edge of mirror. He can see the images of P and Q easily but not the image of R.
PSEB 8th Class Science Solutions Chapter 16 Light 9

Question 17.
(a) Find out the position of image of an object situated at A in plane mirror.
(b) Can Paheli at B see this image ?
(c) Can Bhoojo at C see this image ?
(d) When Paheli moves from B to C where does the image of A move ?
PSEB 8th Class Science Solutions Chapter 16 Light 10
Answer:
PSEB 8th Class Science Solutions Chapter 16 Light 11
(a) The image of A in the mirror will be as far behind as the object in front of the mirror.
(b) Yes, Paheli can see the image.
(c) Yes, Bhoojo can see this image.
(d) When Paheli moved from B to C, the image of A will not move forward.

PSEB 8th Class Science Solutions Chapter 16 Light

PSEB Solutions for Class 8 Science Light Important Questions and Answers

Multiple Choice Questions

Question 1.
The plane and polished surface that returns light falling on it in the same direction or any other direction is called:
(a) Lens
(b) Prisom
(c) Mirror
(d) Kaleidoscope.
Answer:
(c) Mirror.

Question 2.
How many colours are present in white sunlight ?
(a) 2
(b) 5
(c) 7
(d) 3.
Answer:
(c) 7.

Question 3.
What is the most convenient distance for reading by a normal eye ?
(a) 10 cm
(b) 25 cm
(c) 15 cm
(d) 20 cm.
Answer:
(b) 25 cm.

Question 4.
When you see in dim light the size of your pupil becomes:
(a) Small
(b) Large
(c) Neither small nor large
(b) Very small.
Answer:
(b) Large.

Question 5.
The Phenomena of splitting of light into its constituent colours is called:
(a) Reflection
(b) Refraction
(c) Dispersion
(b) Combination.
Answer:
(c) Dispersion.

Question 6.
The eye lenses focuses:
(a) On Cornea
(b) On Retina
(c) On Iris
(d) None of these.
Answer:
(b) On Retina.

PSEB 8th Class Science Solutions Chapter 16 Light

Very Short Answer Type Questions

Question 1.
Can any one see in the dark ?
Answer:
No.

Question 2.
It is what that helps us to see objects ?
Answer:
Light.

Question 3.
Name any two Luminous bodies.
Answer:

  1. Sun
  2. Electric bulb.

Question 4.
Is moon a luminous or a non-luminous body ?
Answer:
Non-luminous.

Question 5.
Where is a reflection seen ?
Answer:
In mirror.

Question 6.
Which surface can act as a mirror ?
Answer:
Any polished and smooth surface.

PSEB 8th Class Science Solutions Chapter 16 Light

Question 7.
If you are standing in front of a mirror and observing your own image what is the relation between the distance of the object and the image from the mirror ?
Answer:
Distance of object from the mirror = Distance of image from the mirror.

Question 8.
What is the angle of reflection, when a ray of light is incident normally on a plane mirror ?
Answer:
Zero.

Question 9.
Name two objects which split white light into many colours.
Answer:
Water bubbles, surface of CD, prism.

Question 10.
Name the seven colours of light.
Answer:
Red, orange, yellow, green, blue, indigo and violet.

Question 11.
Give one example of dispersion of light in nature.
Answer:
A rainbow.

Question 12.
Is focal length of eye lens fixed ? If not, why ?
Answer:
No. Eye lens has variable focal length. Focal length of Eye lens varies due to action of ciliary muscles.

PSEB 8th Class Science Solutions Chapter 16 Light

Question 13.
What is the function of sclerotic in human eye ?
Answer:
Sclerotic provides a solid shape to the eye and protects it from external injuries.

Question 14.
What is the function of ciliary muscles in human eye ?
Answer:
Eye lens is held in its position by ciliary muscles. Ciliary muscles help the eye lens to change its focal length by adjusting its curvature.

Question 15.
What is the function of rods on the retina ?
Answer:
Rods are sensitive to intensity of light. The more the intensity of light, more are they excited.

Question 16.
What are cones ?
Answer:
Cones on retina are sensitive to different colours. If cones are absent or insufficient, the person is colour blind.

Question 17.
What is basic cause of colour blindness ?
Answer:
It is due to absence or insufficient number of cones on the retina. Seeing sun or towards it during solar eclipse may cause colour blindness.

Question 18.
Why cats and bats are able to see at night ?
Answer:
They have very large number of rods on retina. Hence, they are able to see even a small quantity of light.

Question 19.
At what rate the images pass one after the other on a cinema screen ?
Answer:
25 or more per second.

PSEB 8th Class Science Solutions Chapter 16 Light

Question 20.
What type of photosensitive cells are present on the retina of the eye ?
Answer:
Cones and rods.

Question 21.
Name one device that can be used by short statured person to see over the head of a crowd.
Answer:
Periscope.

Question 22.
Find out the letters of English alphabet or any another language known to you in which the image formed in a plane mirror appears exactly like the letter itself.
Answer:
PSEB 8th Class Science Solutions Chapter 16 Light 12

Short Answer Type Questions

Question 1.
What are luminous and non-luminous bodies ? Give examples.
Answer:
Luminous Bodies.
Those objects which emit their own light, are called luminous bodies e.g. sun, stars, burning fire, radium etc.

Non-luminous Bodies.
Those objects which do not emit light of their own but shine due to other luminous bodies, are called Non-Luminous bodies. They are visible only, when light falls on them.
e.g. Moon, earth and other planets, things in a room.

Question 2.
How do we see objects ?
Answer:
When light from a light source falls on any object, it is scattered by it. The scattered light enters our eyes to form the image of the object and the object is, thus, seen.

Question 3.
Give the conditions necessary for seeing an object.
Answer:
Conditions for Seeing Objects. To see an object, the following three conditions are required to be satisfied:

  1. Source of light to make object visible
  2. The object
  3. Eye sight.

Question 4.
What is a virtual image ? Give one situation where a virtual image is formed.
Answer:
Virtual image.
An image, which cannot be obtained on a screen, is called a virtual image. Virtual image is formed when reflected rays do not actually meet at a point.
Image formed in a plane mirror is always virtual.

PSEB 8th Class Science Solutions Chapter 16 Light

Question 5.
What do you understand by lateral inversion ?
Answer:
Lateral Inversion.
In a plane mirror, the right side of the object becomes left side of the image and the left side of the object becomes right side of the image. That is the image is sideways inverted. This phenomenon, is called lateral inversion.

Question 6.
State the laws of reflection of light.
Answer:
Laws of reflection.
The reflection at the smooth surface is found to obey the following two laws, called the laws of reflection.

  1. The angle of incidence is equal to the angle of reflection
    i.e., ∠i – ∠r.
  2. The incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

Question 7.
What is diffused reflection and regular reflection ?
Answer:
Diffused Reflection.
It takes place when the surface is not smooth or polished e.g., wall, paper. The rays reflected from an uneven surface are not parallel but scattered in all directions and such reflection is called diffused reflection.
PSEB 8th Class Science Solutions Chapter 16 Light 13
Regular Reflection.
It takes place when a beam of light falls on a smooth and polished surface, e.g., a mirror. The reflected rays from an even surface form a clear image.

Question 8.
What do you understand by reflection of light ?
Answer:
Reflection of light.
When a ray of light falls on a mirror, or polished surface, it is sent back in a particular direction obeying certain laws of reflection. This phenomenon is known as reflection of light.

Question 9.
Define the following:
1. Reflected Ray
2. Angle of reflection.
Answer:
1. Reflected Ray. A streak of light, starting from the mirror, is called reflected ray.

2. Angle of reflection. The angle made by the reflected ray with the normal at the point of incidence is called angle of reflection. It is represented by r.

Question 10.
If incident ray strikes the mirror at 90°, what will be the angle of reflection ?
Answer:
If incident ray strikes the mirror normally, then after reflection, the ray will come back along the same path. As the angle of incidence is zero, so angle of reflection will also be zero.

PSEB 8th Class Science Solutions Chapter 16 Light

Question 11.
Define the term ‘Dispersion of light.’
Answer:
Dispersion of Light.
When a beam of light passes through a prism, the white light splits into seven colours. This phenomenon of splitting of white light into its constituent colours by a prism is known as dispersion of light.

Question 12.
While passing through a prism, why does the white light split into seven colours.
Answer:
The rays of different colours pass smoothly through air but when passing through a prism they have to change the speed according to the angle of prism. Different colours have different wave lengths and thus, they choose different paths. So, the spectrum is seen.

Question 13.
What is myopia ?
Answer:
Myopia.
The focal length of the eyes of some people is very small. Therefore, the image of distant object is not formed on their retina but it is formed at a point in front of the retina. In other words, they cannot see distant objects. This defect of vision is called Myopia or short sightedness.

Question 14.
What is hypermetropia ?
Answer:
Hypermetropia.
When people grow old, the muscles of the eyes lose their strength to control the curvature of the lens. As a result, the image of nearby objects is not formed at the retina, but it is formed behind the retina. So these people cannot see nearby objects clearly. This defect of vision is called Hypermetropia or Farsightedness.

Question 15.
Explain in short perception of colour.
Answer:
Perception of colour.
Human eye contains large number of rods and cones which are sensitive to light. Rods respond to the intensity of light and cones respond to the colour of the light. If cone cells are absent in the eye then such a person is colour blind. With the help of cones cells one can perceive colour. This is called perception of colour.

Question 16.
The following picture shows the reflection of light:
PSEB 8th Class Science Solutions Chapter 16 Light 14
(A) Name the ray AO
(B) Name the ray OB
(C) Find the value of angle x
Answer:
(A) Name of ray AO = Incident7 ray
(B) Name the ray OB = Reflected ray
(C) Value of angle x = 30° because angle of incidence is equal to angle of reflection.

Long Answer Type Questions

Question 1.
Differentiate between a real image and a virtual image.
Answer:
Differences between a real image and a virtual image.

Real Image Virtual Image
1. The real image is formed when the rays of light actually meet after reflection. 1. The image formed is virtual when the rays of light don’t meet after reflection.
2. Real image can be obtained on the screen. 2. Virtual image cannot be obtained on the screen.
3. Real image is always inverted. 3. Virtual image is always erect.

Question 2.
State the characteristics of image formed by a plane mirror.
Answer:
Characteristics of image formed in a plane mirror.

  1. The image is as far behind the plane mirror, as the object is in front of it.
  2. The image is laterally (sideways) inverted.
  3. The image is of the same size as that of the object.
  4. The image formed in a plane mirror is virtual, erect and of the same size as the object.
  5. The image formed in a plane mirror cannot be obtained on the screen.

PSEB 8th Class Science Solutions Chapter 16 Light

Light PSEB 8th Class Science Notes

  • Light is a form of energy.
  • Light travels in a straight line.
  • Light enables us to see the objects around us.
  • Light reflected from the objects falls into our eyes and thus, we can see the objects.
  • Those objects, which emit their own light, are called luminous objects.
  • The objects, which do not emit light of their own, but they reflect light falling on them, are called non-luminous objects.
  • Polished or shiny surfaces reflect light.
  • A mirror changes the direction of light, that falls on it.
  • Angle of incidence (∠i) is always equal to angle of reflection (∠r).
  • Incident ray, normal at the point of incidence and reflected ray all lie in the same plane.
  • Number of reflections can be achieved with the mirrors arranged at an angle.
  • When the light passes through a prism, it undergoes dispersion and white light splits up into seven colours.
  • The spectrum of sunlight consists of seven colours – Violet, Indigo, Blue, Green, Yellow, Orange and Red. These seven colours can be remembered by term VIBGYOR. Rainbow is a natural phenomenon of dispersion.
  • Human eye is a sensitive organ, which enables us to see the objects around us.
  • A human eye has a convex lens, whose focal length is adjusted by ciliary muscles. Reflections can be parallel or regular and diffused.
  • Cones and rods are two types of nerve ending found in blind spot of the eye.
  • Two types of resources for visually challenged persons are : Non-optical aids and optical aids.
  • Braille system is one of the most important and popular resource , for visually challenged persons.
  • Reflection of Light: The change in direction of light by a shiny surface is called reflection of light.
  • Screen: The white sheet or surface on which, the image is obtained.
  • Regular Reflection: Reflection taking place from a polished and regular surface.
  • Scattering of Light: Diffusing of light in all directions.
  • Diffused Reflection: The light rays reflected from non-polished or irregular surface.
  • Incident Ray: A ray of light from a source of light falling on a given surface.
  • Kaleideoscope: An instrument based on multiple reflection used to create new designs.
  • Mirror: A smooth and shiny surface.
  • Normal: The perpendicular to the polished surface (mirror) at the point of incidence.
  • Source of Light: An object which emits light.
  • Real Image: The image formed, when incident rays after reflection actually meet at a point.
  • Virtual Image: The image formed, when incident rays after reflection do not actually meet, but appear to meet at a point.
  • Angle of Incidence: The angle between the incident ray and the normal.
  • Angle of Reflection: The angle between the reflected ray and the normal.
  • Power of Accommodation is the ability of the eye to focus the distant objects as well as the nearby objects, at the retina by changing the focal length or converging power of its lens.
  • Least Distance of Distinct Vision is the minimum distance at which object must be placed so that a normal eye may see the object clearly without any strain on the eye. It is about 25 cm for normal eye.
  • Myopia or Short-Sightedness means a person can see only nearby objects clearly, but cannot see distant objects. This defect is removed by using spectacles with concave lens.
  • Hypermetropia or Long-sightedness: A person suffering from this defect can see distant objects clearly but cannot see nearby objects. The defect is removed by using spectacles fitted with convex lens.
  • Persistence of Vision: Impression of an image on the retina lasts for 0.1 sec after the removal of the object. This effect is known as persistence of vision.
  • Dispersion of Light is the splitting of white or some other light into its constituents.
  • Perception of Colour: Human eye contains a large number of cell rods and cones which are sensitive to light. Rods respond to the intensity of light and cones respond to the colour of the light. If cone cells are absent in the eye, then such a person is colour blind.

Punjab State Board PSEB 8th Class Science Book Solutions Chapter 16 Light Textbook Exercise Questions and Answers.