# NCERT Solutions for Class 7 Science Chapter 15 Light

These NCERT Solutions for Class 7 Science Chapter 15 Light Questions and Answers are prepared by our highly skilled subject experts to help students while preparing for their exams.

## Light NCERT Solutions for Class 7 Science Chapter 15

### Class 7 Science Chapter 15 Light Textbook Exercise Questions and Answers

Question 1.
Fill in the blanks:
a. An image that cannot be obtained on a screen is called ………………..
b. Image formed by a convex ……………….. is always virtual and smaller in size.
c. An image formed by a ……………….. mirror is always of the same size as that of the object.
d. An image which can be obtained on a screen is called a ……………….. image.
e. An image formed by a concave ……………….. cannot be obtained on a screen.
a. virtual
b. mirror
c. plane
d. real
e. lens

Question 2.
Mark “T” if the statement is true and “F” if it is false.
a. We can obtain an enlarged and erect image by a convex mirror.
b. A concave lens always forms a virtual image.
c. We can obtain a real, enlarged and inverted image by a concave mirror.
d. A real image cannot be obtained on screen.
e. A concave mirror always forms a real image.
a. False
b. True
c. True
d. False
e. False

Question 3.
Match the items given in Column I with one or more item of Column II.

 Column I Column II 1. A plane mirror (a) Used as a magnifying glass. 2. A convex mirror (b) Can form image of objects spread over a large area. 3. A convex lens (c) Used by dentists to see enlarged image of teeth. 4.A concave mirror (d) The image is always inverted and magnified. 5. A concave lens (e) The image is erect and of the same size as the object. (f) The image is erect and smaller in size than the object.

1. (e),
2. (b)
3. (a),
4. (c),
5. (f)

Question 4.
State the characteristics of the image formed by a plane mirror.
Characteristics of the image formed by a plane mirror are:

• The image formed is virtual.
• The image is laterally inverted.
• It is of the same size as the object.
• The image is situated at the same distance from the mirror as the object.
• The image is erect.

Question 5.
Find out the letters of English alphabet or any other language known to you in which the image formed in plane mirror appears exactly like the letter itself. Discuss your findings.
A, H, I, M, O, T, U, V, W, X, Y are the letters of English alphabet in which the image formed in a plane mirror appears exactly like the letter itself.

Question 6.
What is a virtual image? Give one situation where a virtual image is formed.
A virtual image is an image that cannot be obtained or formed on a screen. The image formed in plane mirror cannot be obtained on a screen because this is a virtual image.

Question 7.
State two differences between a convex and a concave lens.
Difference between convex lens and a concave lens are:

 Convex lens Concave lens 1. Convex lens can form both real and virtual images. 1. Concave lens always forms a virtual image. 2. It can form magnified image. 2. Image is always diminished in size.

Question 8.
Give one use each of a concave and a convex mirror.
Convex mirrors are used in roads and large buildings to allow people to see what is around the bend. Concave mirrors are used in reflecting telescopes and also as a magnification tool.

Question 9.
Which type of mirror can form a real image?
A concave mirror can form a real image of an object. The nature of the image depends on the distance of the object from the concave mirror.

Question 10.
Which type of lens forms always a virtual image?
A concave lens always forms a virtual image of an object.

Choose the correct option in questions 11-13.

Question 11.
A virtual image larger than the object can be produced by a
a. concave lens
b. concave mirror
c. convex mirror
d. plane mirror
c. concave mirror.

Question 12.
David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then the distance between David and his image will be
a. 3 m
b. 5 m
c. 6 m
d. 8 m
c. 6 m

Question 13.
The rearview mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rearview mirror, the image of a truck parked behind the car. The speed at which the image of the truck appears to approach the driver will be
a. 1 m/s
b. 2 m/s
c. 4 m/s
d. 8 m/s
c. 4 m/s

NCERT Extended Learning Activities and Projects

Question 1.
Play with a mirror. Write your name with a sketch pen on a thin sheet of paper, polythene or glass. Read your name on the sheet while standing in front of a plane mirror. Now look at your image in the mirror.
Hint:
Do it yourself.

Question 2.
A burning candle in water: Take a shoebox, open on one side. Place a small lighted candle in it. Place a clear glass sheet (roughly 25 cm x 25 cm) in front of this candle. Try to locate the image of the candle behind the glass sheet. Place a glass of water at its position. Ask your friends to look at the image of the candle through the sheet of glass. Ensure that candle is not visible to your friends. Your friends will be surprised to see the candle burning in water. Try to explain the reason.

Hint:
When we put a glass filled with water in that position, then the image of the burning candle appears to be in it. This is all because plane glass sheet is transparent so the image of burning candle is fonned behind the glass sheet.

Question 3.
Make a rainbow. Try to make your own rainbow: You can try this project in the morning or in the evening. Stand with your back towards the sun. Take a hosepipe or a water pipe used in the garden. Make a fine spray in front of you. You can see different colours of rainbow in the spray.
Hint:
Do it yourself.

Question 4.
Visit a laughing gallery in some science centre or a science park or a village mela. You will find some large mirrors there. You can see your distorted and funny images in these mirrors. Try to find out the kind of mirror used there.
Hint:
Spherical mirrors are used to form funny images. This is because normal plane mirrors produce a perfect image whereas we need distorted, funny images which can be only produced by curved mirrors.

Question 5.
Visit a nearby hospital. You can also visit the clinic of an ENT specialist or a dentist. Request the doctor to show you the mirror used for examination of ear, nose, throat and teeth. Can you recognise the kind of mirror used in these instruments?
Hint:
Concave mirrors are used by dentists because at a short range, the concave mirror produces magnified, upright images of face. By these mirrors, dentists can examine the patient more easily.

Question 6.
Roleplay: Here is a game that a group of children can play. One child will be chosen to act as object and other will act as the image of the object. The object and the image will sit opposite to each other. The object will make movements, such as raising a hand, touching an ear, etc. The image will have to make the correct movement following the movement of the object. The rest of the group will watch the movements of the image. If the image fails to make the correct movement, she/he will be retired. Another child will take her/ his place and the game will continue. A scoring scheme can be introduced. The group that scores the maximum will be declared the winner.
Hint:
Do it yourself.

Activity 1

Objective: To show that the image formed in a plane mirror is erect and virtual.
Materials Required: A large-sized plane mirror and a candle.
Procedure:

• Place a lighted candle in front of a plane mirror. Try to see the flame of the candle in the mirror. It appears as if similar candle is placed behind the mirror.
• The candle, which appears behind the mirror, is the image of the candle formed by the mirror. The candle itself is the object.
• Now move the candle to different positions in front of the mirror. Observe the image in each case.
• Now place a vertical screen behind the mirror. Try to obtain the image of the candle on this screen. The image of the candle cannot be obtained on the screen.
• Now place the screen in front of the mirror. The image of the candle cannot be obtained on the screen in this case also.

Observations: The image of candle cannot be obtained on a screen in either case.
Conclusion: This happened because the image formed in a plane mirror is erect and virtual.

Activity 2

Objective: To show that the image formed in a plane mirror is laterally inverted.
Materials Required: A large size plane mirror.
Procedure:

• Stand in front of a big plane mirror and observe your image in the mirror.
• Lift your right hand, then the image lifts its left hand.
• Now lift your left hand and then the image appears to lift its right hand.

Left hand appears on the right side in the image

Observations: When you see yourself in the plane mirror, your right side part of the body becomes left and left side part of the body become right.
Conclusion: When an object is placed in front of a plane mirror, the lateral sides of the object are changed and this is called lateral inversion.

Spherical Mirrors: When the mirror is a part of a sphere having a curved reflecting surface, it is called a spherical mirror. Spherical mirrors are classified based on the shape of their reflecting surface in two types- concave and convex. When a part of sphere is cut, the inside part is called the concave side, while the outside part is called the convex side. A common example is a stainless steel spoon whose inside part behaves as a concave mirror and outer part behaves as a convex mirror.

i. Concave Mirror: It is a spherical mirror whose reflecting surface is curved inwards. In case of concave mirror, the image is usually inverted, real and smaller than the object. When the object is kept too close to the concave mirror, the image is erect, virtual and larger than the object.

Uses of Concave Mirror:

• Concave mirror is used by dentists to focus a beam of light to see inside a patient’s mouth or ears.
• Concave mirror is used in solar furnaces, because rays of sunlight converge at a point once they reflect from the concave mirror, generating heat.
• Concave mirror is used as barber’s mirror because it shows a larger image when object is too close to the mirror. It helps in getting a closer shave.
• It is also used in reflectors of torches and headlights of vehicles.

Activity 3

Objective: To show that a concave mirror forms a real image of the sun.
Materials Required: A concave mirror and a sheet of paper.
Procedure:

• Take a concave mirror.
• Hold it facing the sun.
• Try to get the light reflected by the mirror on a sheet of paper.
• Adjust the distance of the paper until you get a sharp bright spot on it.
• Hold the mirror and the sheet of paper steady for a few minutes.

Observation: While doing this activity, the paper starts burning because the real image of sun is formed by the concave mirror.
Conclusion: Concave mirror forms real images.

ii. Convex Mirror: It is a spherical mirror whose reflecting surface is curved outwards. In case of convex mirror, the image is always erect, virtual and smaller than the object.

Uses of Convex Mirror:

• Convex mirror is used in rear view mirrors of vehicles, as it shows smaller images from a bigger field of view.
• Convex mirror is also used on hairpin bends of roads to see the vehicles coming from the other side of the bend.

Activity 4

Objective: To study the images formed by a convex mirror.
Materials Required: Convex mirror, mirror stand, candle and white paper screen.
Procedure:

• Mount a convex mirror on a mirror stand and place it on a table.
• Place a screen made from white paper at a distance in front of it.
• Place a lighted candle in between the mirror and the screen.
• Move the screen to see if any image is formed on it. No image is seen on the screen.
• Now look into the convex mirror. An erect and smaller image is seen in the mirror.
• Slowly move the candle away from the mirror and keep observing the nature of the image formed in the mirror.

Observation: In convex mirror, the images of candle are smaller and erect at all distances.
Conclusion: Convex mirror always forms erect and small-sized images.

Lens: A lens is a part of a reflecting material like glass or plastic but transparent from both sides. Lenses are unlike mirrors that have a reflecting surface only on one side. Depending upon its shape, a lens can be categorised as follows:
i. Convex lens: This lens is thinner at the edges and thicker in the centre. It is a converging lens as it converges light rays. It can form real, inverted and diminished image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified.

A magnifying glass is a convex lens which is used to make small things look bigger by producing their magnified (enlarged) images.

ii. Concave lens: It is the lens that is thinner at the centre than at the edges. It is a diverging lens as it diverges light rays. Image formed is always virtual, erect and diminished.

Refraction of light ray through lenses
(a) Diverging action of a concave lens
(b) Converging action of a convex lens

Uses of Lenses: Lenses are used in microscopes, telescopes and cameras. They are used in astronomical telescopes. They are also used in reading glasses.

Components of White Light: The white light or sunlight is composed of seven colours. These colours in order are VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange and Red). When white light passes through a glass prism, it breaks down into its component colours. This is the reason why rainbows show all the colours of the white light. Several phenomena explain that sunlight or white light is composed of seven colours.

Activity 5

Objective: To show that the sunlight consists of seven colours.
Materials Required: A glass prism and a white sheet of paper.
Procedure:

• Take a glass prism
• Allow a narrow beam of sunlight through a small hole in the window of a dark room to fall on one face of the prism.
• Let the light coming out of the other face of the prism fall on a white sheet of paper.

Observation: When the light coming out of the other face of the prism falls on a white sheet then a band of seven colours appear on the screen.
Conclusion: When a beam of white light is passed through a glass prism, then the white light splits to form a band of seven colours on a white screen. Hence, sunlight is made up of seven colours.

i. Dispersion of light: The splitting up of white light into seven colours on passing through a transparent medium, like a glass prism, is called dispersion of light. The formation of spectrum of seven colours on the other side of the prism shows that white light is a mixture of seven colours.

ii. Rainbow: The rainbow is an arch of seven colours in the sky. The rainbow is produced by the dispersion of sunlight by tiny rain drops suspended in the atmosphere that behave like minute prisms.

iii. Newton’s Disc: It is a circular disc on which seven colours of the rainbow are painted on seven different sectors. When the disc is rotated at certain speeds, it appears to be white. This shows how the seven colours combine to make the white light.

### Class 7 Science Chapter 15 Light Additional Important Questions and Answers

Question 1.
What is light?
Light is a form of energy which helps us in getting the vision.

Question 2.
How many colours are there in the visible spectrum?
Seven.

Question 3.
Which types of images can be formed on a cinema screen?
Real images.

Question 4.
Is the image of an object in a plane mirror virtual or real?
Virtual.

Question 5.
Define the image of an object.
Due to the reflection of light, the impression of an object formed in a mirror is called the image of the object.

Question 6.
What is virtual image?
The image that cannot be captured on a screen is called a virtual image.

Question 7.
What is real image?
Image that can be captured on a screen is known as real image.

Question 8.
Give an example of real image.
In a camera, images are real and can be captured on the negative, which acts as a screen.

Question 9.
What happens when light rays are incident on a concave lens?
When light rays are incident on a concave lens, they bend outwards from the lens or diverge.

Question 10.
How is a rainbow formed in the sky?
A rainbow is formed by the dispersion of the sun’s rays through raindrops into seven colours.

Question 11.
What is VIBGYOR?
It represents the order of seven colours in the rainbow, i.e., violet, indigo, blue, green, yellow, orange and red.

Question 12.
What happens when light falls on a polished or a shiny surface?
When light falls on a polished or a shiny surface, it gets reflected.

Question 13.
What makes things visible to us?
Objects are visible only when light reflected from them reaches our eyes.

Question 14.
What is reflection of light?
Bouncing back of a light ray after hitting any surface is known as reflection of light.

Question 15.
Why we are not able to see the candle flame through a bent pipe?
We are not able to see the candle flame through a bent pipe because light travels along straight lines.

Question 16.
How can we change the path of light?
We can change the path of light by keeping any shiny or polished or reflecting material in the path of the light beam.

Question 17.
What type of image does the outer side of a spoon show?
The outer surface of a spoon acts like a convex mirror.

Question 1.
What do you mean by lateral inversion? Give a suitable example.
When an object is placed in front of a plane mirror, then the right side of the object appears to be the left side of the image and the left side of the object appears to be the right side of the image. This change of sides of an ‘object’ and its ‘mirror image’ is called lateral inversion. For example, the word ‘AMBULANCE’ on an ambulance is written in mirror writing

Question 2.
State any two uses each of concave mirror and convex mirrors.
Concave mirrors:

1. Concave mirrors are used as reflectors in torches, headlights of vehicles.
2. Concave mirrors are used by the dentist to see an enlarged image of teeth.

Convex mirrors:

1. Convex mirrors are used as rearview mirrors or side-view mirrors in automobiles such as cars, scooter, trucks, buses, etc., to see the traffic coming from behind.
2. Convex mirrors are used as staircase mirrors on the double-decker buses.

Question 3.
Why is a concave mirror called a converging mirror and a convex mirror called a diverging mirror?
Concave mirror is called a converging mirror because it converges the parallel rays of light that fall on the mirror at a point called focus. Convex mirror is called a diverging mirror because parallel rays of light falling on it diverge after reflection.

Question 4.
Distinguish between concave mirror and convex mirror.

 Concave mirror Convex mirror 1. It can form real and virtual image. 1. It always forms a virtual image. 2. It can form magnified image. 2. It always forms a diminished image. 3. It can form both erect and inverted image. 3. It always form an erect image. 4. It reflects light from its inner shining surface. 4. It reflects light from its outer shining surface.

Question 5.
Why is convex mirror used as side mirror in scooters?
We can recognise that the mirrors used as side mirror in scooters are convex mirrors. Convex mirror can form erect images of object spread over large area, so helps the drivers to see the traffic behind them.

Question 6.
What is the difference between a virtual image and a real image?
The following are the main differences between a real image and a virtual image.

• A real image can be obtained on a screen whereas a virtual image cannot be obtained on a screen.
• A real image is always inverted whereas a virtual image is always erect.
• A real image is formed when the rays of light, after reflection, actually meet at some point whereas a virtual image is formed when the rays of light, after reflection, appear to meet at a point but do not actually meet.

Question 7.
Why do we need a shiny surface for reflection?
The extent of reflection depends upon the shine and smoothness of the surface. More the shine and smoothness of the surface, more will be the reflection. That is why, mirrors reflect most of the light falling on them. Hence, for reflection, shiny surfaces are required.

Question 8.
Give the characteristics of images formed by convex and concave lenses.

1. Convex lens: A convex lens can form real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified.
2. Concave lens: A concave lens always forms erect, virtual and smaller image than the object.

Question 9.
What are spherical mirrors? Give their types.
Mirrors having curved surfaces are known as spherical mirrors. Their name is so, because they are considered as a part of a hollow sphere. Mirrors are mainly of two types: concave mirrors and convex mirrors.

a. Concave mirror: Its reflecting surface is curved inwards.
b. Convex mirrors: Its reflecting surface is bulged out.

Question 1.
a. What is a lens? Name the two types of lenses. Name any five things which use lenses.
b. What kind of lens is used as a magnifying glass?
c. Which type of reflector is used in car headlights for producing a parallel beam of light?
a. A lens is a piece of any transparent material bound by two curved surfaces or by one curved and one plane surface. When light rays pass through a lens, they bend and change their direction. The two types of lenses are:

• convex lens, and
• concave lens.

The five things which use lenses are: Spectacles, cameras, microscopes, telescopes and film projectors.
b. Convex lens is used as a magnifying glass.
c. Concave mirrors are used in vehicle headlights to send parallel rays because it allows the light rays to be focused as a single beam and give more power to the light that makes it more efficient for seeing and to be seen by others.

Question 2.
What is Newton’s disc? Why does it appear white when rotated?
A circular disc constructed by Newton which is divided into seven parts painted with seven colours is known as Newton’s disc. The colours painted on disc are those of the spectrum of visible light. When the disc is rotated fast, the colours get mixed together and the disc appears to be whitish. This appearance of white light on rotating disc suggests that the white light consists of seven colours.

Question 3.
Show that the image is at the same distance behind the plane mirror as the object is in front of it.
Take a chessboard. Draw a thick line in the middle of it. Fix a plane mirror vertically on this line. Place a pencil sharpener; at the boundary of the third square counting from the mirror. Note the position of the image. Now shift the object to the boundary of the fourth square. Again note the position of the image. We will find that the image is at the same distance behind the mirror as the object is in front of it.

Image in plane mirror is at the same distance as the object in front of it

Question 4.
Show how light gets reflected from a plane mirror with the help of an activity.
Take a torch. Cover its glass with a chart paper which has three slits. Spread a sheet of chart paper on a smooth wooden board. Fix a plane mirror strip vertically on the chart paper. Now direct the beam of light on the mirror from the torch with slits. Place the torch in such a way that its light is seen along the chart paper on the board. Now adjust its position so that the light from the torch strikes the plane mirror at an angle. We will see that the mirror changes the direction of light that falls on it. Now move the torch slightly to either side. We will see change in the direction of reflected light too. Look into the mirror along the direction of the reflected light. We see the slits in the mirror. This is the image of the slits. This activity shows how light gets reflected from a plane mirror.

Question 5.
Write an experiment to show that the sunlight consists of seven colours.
Take a glass prism. Allow a narrow beam of sunlight through a small hole in the window of a dark room to fall on one face of the prism. Let the light coming out of the other face of the prism fall on a white sheet of paper or on a white wall. We see colours similar to those in a rainbow. This shows that the sunlight consists of seven colours.

Picture-Based Questions

Question 1.
Draw a diagram to show converging nature of convex lens and diverging nature of a concave lens. What do you mean by converging and diverging nature?