# NCERT Solutions for Class 7 Science Light

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NCERT Solutions for Class 7 Science Light, here we are going to give you a summary of NCERT Solutions for Class 7 Science Light with questions and answers from the textbook. In this chapter we will study the properties of light and about the different types of mirrors and different types of lenses. Your all doubts from the chapter NCERT Solutions for Class 7 Science Light will be cleared from this. Here you will find science class 7 chapter 15 light question answer, ncert solutions for class 7 science chapter 16, ncert class 7 science chapter 15 light explanation, class 7 science chapter 15 light notes, class 7 science chapter 15 light extra questions, ncert class 7 science chapter 15 light pdf download, light chapter of class 7 pdf.

## Introduction: NCERT Solutions for Class 7 Science Light

We have seen a  beam of sunlight when it enters a room through a narrow opening or a hole. We have seen light coming out from lamps, bulbs, tube lights, torch, etc. Here we will learn more about light and its properties here. We will also discuss about lenses and different types of mirrors.

## LIGHT TRAVELS ALONG A STRAIGHT LINE

If we look at a candle flame through a straight pipe we are able to see the flame. But if we bend the pipe and then look at the candle flame, we will not be able to look at the flame. This is because flame travels in straight lines.

## REFLECTION OF LIGHT

When light falls on a polished or a shiny surface it changes its direction. For example, a shining stainless steel plate or a shining steel spoon can change the direction of light.  The surface of water can also act like a mirror and change the path of light. Any polished or a shiny surface can act as a mirror and change the direction of light. This change of direction by a mirror is called reflection of light.

### ACTIVITY 1 – NCERT Solutions for Class 7 Science Light

• We take a torch.
• We cover its glass with a chart paper which has three slits. Then we 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.
• We place the torch in such a way that its light is seen along the chart paper on the board.
• Now we adjust its position so that the light from the torch strikes the plane mirror at an angle.

Does the mirror change the direction of light that falls on it?

Look into the mirror along the direction of the reflected light. Do you see the slits in the mirror?

Ans. Yes, this is the image of the slits.

This is how light gets reflected from a plane mirror.

### Activity 2

• We place a lighted candle in front of a plane mirror.
• We try to see the flame of the candle in the mirror.
• It appears as if a 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 we move the candle to different positions in front of the mirror and observe the image in each case.

Was the image upright in each case? Did the flame appear on top of the candle as in the object?

Ans. Yes,  the image was upright in each case. Such an image is called an erect image.

An image formed by a plane mirror is erect and of the same size as the object.

NOTE: In a plane mirror the image is formed behind the mirror. It is erect, of the same size and is at the same distance from the mirror as the object is in front of it.

### Activity 3

• We take a chess board. Let us draw a thick line in the middle of the paper.
• We fix a plane mirror vertically on this line.
• Let us place any small object, such as a pencil sharpener, at the boundary of the third square counting from the mirror.
• Let us note the position of the image. We now shift the object to the boundary of the fourth square. Again we note the position of the image.

Did you find any relation between the distance of the image from the mirror and that of the object in front of it?

Ans. We will find that the image is at the same distance behind the mirror as the object is in front of it.

## RIGHT OR LEFT!

When you see your image in a plane mirror, is it exactly like us? Let us find out.

### Activity 4

• We stand in front of a plane mirror and look at your image.
• Let us raise our left hand.
• Now we touch our right ear.

Which hand does our image raise? Which ear does your hand touch in your image?

Ans. If we observe carefully we will find that in the mirror the right appears left and the left appears right. Only sides are interchanged; the image does not appear upside down.

When the driver of a vehicle ahead of an ambulance looks in her/his rear view mirror, she/he This is why AMBULANCE is written on Ambulance vehicles laterally inverted. We can read AMBULANCE written on it in our rear mirror and give way to it. It is the duty of every one of us to allow an ambulance to pass without blocking its way.

## PLAYING WITH SPHERICAL MIRRORS

Let us find out more about spherical mirrors.

### Activity 5

• We take a stainless steel spoon and bring the outer side of the spoon near our face and look into it.

Do you see your image in it?

Ans. Yes.

Is this image different from what you see in a plane mirror?

Ans. Yes.

Is this image erect? Is the size of the image the same, smaller or larger?

Ans. the image is smaller and erect.

Now we look at your image using the inner side of the spoon.

This time we find that our image is erect and larger in size.

If we increase the distance of the spoon from our face, we may see our image inverted.

The curved shining surface of a spoon acts as a spherical mirror.

Concave mirror: If the reflecting surface of a spherical mirror is concave, it is called a concave mirror.

Convex mirror: If the reflecting surface is convex, then it is a convex mirror.

The inner surface of a spoon acts like a concave mirror, while its outer surface acts like a convex mirror.

### ACTIVITY 6 – NCERT Solutions for Class 7 Science Light

• We take a concave mirror. We hold it facing the sun.
• Let us try to get the light reflected by the mirror on a sheet of paper and adjust the distance of the paper until we get a sharp bright spot on it.
• We hold the mirror and the sheet of paper steady for a few minutes.

Does the paper start burning?

Ans. Yes, the paper starts burning.

The sharp bright spot on the paper is the image of the sun. This image is formed on a screen, i.e. the paper.

Real image: An image formed on a screen is called a real image.

Virtual image:  An image which can not be formed on a screen is called a virtual image.

### ACTIVITY 7

• We fix a concave mirror on a stand. We paste a piece of white paper on a cardboard sheet. This will act as a screen.
• Let us keep a lighted candle on the table at a distance of about 50 cm from the mirror.
• Try to obtain the image of the flame on the screen. For this, move the screen till a sharp image of the flame is obtained. Make sure that, the screen does not obstruct the light from the candle falling on the mirror.
• Now we move the candle towards the mirror and place it at different distances from it. In each case let us try to obtain the image on the screen. Record your observation.

Is this image real or virtual?

Ans.  The image may also be real or virtual.

Is it of the same size as the flame?

Ans. We see that the image formed by a concave mirror can be smaller or larger in size than the object.

Concave mirrors are used for many purposes:

1. Doctors use concave mirrors for examining eyes, ears, nose and throat.
2. Concave mirrors are also used by dentists to see an enlarged image of the teeth.
3. Reflectors of torches, headlights of cars and scooters are concave in shape.

### Activity 8

We repeat Activity 7 now with a convex mirror in place of a concave mirror.

Could you get a real image at any distance of the object from the convex mirror?

Ans.  Convex mirrors can form images of objects spread over a large area.

Did you get an image larger in size than the object?

Ans. No.

## IMAGES FORMED BY LENSES

The magnifying glass is actually a type of a lens.Lenses are widely used in spectacles, telescopes and microscopes.

Convex lenses: Lenses which feel thicker in the middle than at the edges are convex lenses.

Concave lenses: Lenses which feel thinner in the middle than at the edges are concave lenses

### Activity 9 – NCERT Solutions for Class 7 Science Light

• We take a convex lens or magnifying glass.
• Let us put it in the path of sun rays.
• We place a sheet of paper. Adjust the distance between the lens and the paper till we get a bright spot on the paper.
• We hold the lens and the paper in this position for a few minutes.
• Does the paper begin to burn?

Ans. Yes

Now replace the convex lens with a concave lens.

Do you see a bright spot on the paper this time, too?

Ans. No.

### Activity 10

• We take a convex lens and fix it on a stand as we did with the concave mirror.
• We place it on a table. Place a lighted candle at a distance of about 50 cm from the lens.
• We try to obtain the image of the candle on a paper screen placed on the other side of the lens.

What kind of image did you get, real or virtual?

Ans. Virtual

Did you get in any position of the object an image which was erect and magnified?

Ans. No

Could this image be obtained on a screen?

Ans. No

Is the image real or virtual?

Ans. Virtual.

This is how a convex lens is used as a magnifying glass.

We do a similar activity with a concave lens.

We find that the image formed by a convex lens is always virtual, erect and smaller in size than the object.

## SUNLIGHT — WHITE OR COLOURED?

Sunlight is a mixture of different colours.

### ACTIVITY 11 – NCERT Solutions for Class 7 Science Light

• We take a glass prism.
• We 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. But sunlight is considered to be white. Thus white light is of seven colours.

### Activity 12

• We take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint the seven rainbow colours on these segments.
• We make a small hole at the centre of the disc. Fix the disc loosely on the tip of a refill of a ball pen. Ensure that the disc rotates freely.
• Rotate the disc in the daylight.

When the disc is rotated fast, the colours get mixed together and the disc appears to be whitish. Such a disc is popularly known as Newton’s disc.

## Conclusion : NCERT Solutions for Class 7 Science Light

In NCERT Solutions for Class 7 Science Light we have learnt about light and its properties, importance and many other things. Human beings have evolved and have used light to their own uses. Without knowing the properties of light, life would be doomed. The mirrors, lenses all work on the properties of light. Light is a key element in our everyday lives. Light can carry both energy and information, about which we will study more in higher classes. Let us now move on to NCERT Solutions for Class 7 Science Light : Exercises.

## Exercises – NCERT Solutions for Class 7 Science Light

1. Fill in the blanks:

(a) An image that cannot be obtained on a screen is called a virtual image.

(b) Image formed by a convex mirror is always virtual and smaller in size.

(c) An image formed by a plane 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 real image.

(e) An image formed by a concave lens cannot be obtained on a screen.

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. (T/F)

Ans. False

(b) A concave lens always forms a virtual image. (T/F)

Ans. True

(c) We can obtain a real, enlarged and inverted image by a concave mirror. (T/F)

Ans. True

(d) A real image cannot be obtained on a screen. (T/F)

Ans. False

(e) A concave mirror always forms a real image. (T/F)

Ans. True

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

4. State the characteristics of the image formed by a plane mirror.

Ans. The characteristics of the image formed by a plane mirror are:

1. Erect
2. Laterally inverted
3. Same size
4. Distance is the same from mirror as the distance between the object and the mirror.
##### 5. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.

Ans. the letters of English alphabet which the image formed in a plane mirror appears exactly like the letter itself are:

A, H, I, M, O, T, U, V, W, X, Y.

6. What is a virtual image? Give one situation where a virtual image is formed.

Ans.  If an image can not be obtained on a screen it is called a virtual image.

7. State two differences between a convex and a concave lens.

Ans.

8. Give one use each of a concave and a convex mirror.

Ans. One use each of a concave and a convex mirror:

Concave mirror:

• Used by dentists to see cavities in the tooth.

Convex mirror:

• Used in rear mirrors in cars.because they enable the driver to view a much larger area than a plane mirror.

9. Which type of mirror can form a real image?

Ans. Concave mirror can form a real image.

10. Which type of lens always forms a virtual image?

Ans. Concave lens always forms a virtual image.

##### Choose the correct option in questions 11–13

11. A virtual image larger than the object can be produced by a

(i) concave lens (ii) concave mirror (iii) convex mirror (iv) plane mirror

Ans. (ii) concave mirror

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

(i) 3 m (ii) 5 m (iii) 6 m (iv) 8 m 13.

Ans. (iii) 6 m

13. The rear view 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 rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be

(i) 1 m/s (ii) 2 m/s (iii) 4 m/s (iv) 8 m/s

Ans. (iii) 4 m/s