Class 7 Science Light Chapter Notes

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Light

5.1 Introduction
Light is a source of energy which enables us to see the world surroundings us. Light itself is not visible. Without light, we can not even imagine life on earth.

5.2 Light travels along a straight line

First try to see the lighted candle through a straight pipe and then through the bent pipe. You will not be able to see the candle through the bent pipe. This activity shows light travels along straight lines.

5.3 Reflection of Light
One way to change the direction of light is to let it fall on a shiny surface. 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.

Reflection: The bouncing back of light into the same medium is called reflection of light. An incident ray OA strikes the mirror surface at A. AB is the reflected ray. AM is the normal to the mirror surface.

Angle of incidence: The angle made by incident ray with the normal is called angle of incidence. Ði is the angle of incidence.

Angle of reflection: The angle made by reflected ray with the normal is called angle of reflection. Ðr is angle of reflection.
Laws of reflection
i) The incident ray, the reflected ray and the normal all lie in the same plane.
ii) The angle of incidence is equal to the angle of reflection i.e. Ði =Ðr

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 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.

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. In a plane mirror, image is laterally inverted. That is right becomes left and left becomes right.

5.4 Types of Images
There are two types of images
i) Real image ii) Virtual image

Real Image: The image which can be obtained on a screen is called real image. Concave mirror forms real image.

Virtual Image: The image which cannot be obtained on a screen is called virtual image. Plane mirror and convex mirror forms virtual image.

5.5 Spherical Mirrors
The most common example of a curved mirror is a spherical mirror. If the reflecting surface of a spherical mirror is concave, it is called a concave 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.

In the following videos we have explained you about the concept of Light and its properties. Watch this video to understand the concepts and also post your questions and comments in the comments section

Part 1

The image formed by a concave mirror can be smaller or larger in size than the object. The image may also be real or virtual.

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

Image formed by convex mirrors are virtual, erect and smaller in size. Can you recognise the mirrors used as side mirrors in scooters? These are convex mirrors. Convex mirrors can form images of objects spread over a large area. So, these help the drivers to see the traffic behind them.

5.6 Terms related to Spherical Mirrors

Consider a spherical mirror as shown in the given figure.

i) Centre of Curvature: The centre of the sphere of which mirror is a part is called centre of curvature. In the diagram, C is the centre of curvature.

ii) Radius of curvature: The radius of the sphere of which mirror is a part is called radius of curvature. In the diagram, PC is called radius of curvature.

iii) Pole: The centre of the reflecting surface of the mirror is called pole. In the diagram, P is called pole of the mirror.

iv) Focus: The point where all rays meet or appears to meet after reflection is called Foc In the diagram, F is called focus.

v) Focal length: The distance between Pole to the focus is called focal length. In the diagram, PF is called focal length.

5.7 Mirror Equation
The mirror equation is given by
1/v + 1/u = 1/f

v is called image distance 

u is called object distance 

f is called focal length

5.8 Lenses

Lens is an optical device used for refracting and transmitting light. You might have seen a magnifying glass. It is used to read very small print. The magnifying glass is actually a type of lens. Lenses are widely used in spectacles, telescopes and microscopes.
Types of lenses:

i) Concave Lens: Concave lens is thinner at middle.

ii) Convex lens: Convex lens is thicker at middle.

A convex lens converges (bends inward) the light generally falling on it. Therefore, it is called a converging lens. On the other hand, a concave lens diverges (bends outward) the light and is called a diverging lens.

A convex lens forms magnified image.

A concave lens forms virtual, erect and diminished image.

5.9 Rainbow

There are seven colours in a rainbow, though it may not be easy to distinguish all of them. These are — red, orange, yellow, green, blue, indigo and violet.

The prism splits sunlight into seven colours. The process of splitting of white light into seven colours is called dispersion.

5.10 Newton’s Disc

Take a circular cardboard disc of about 10 cm diameter. Divide this disc into seven segments. Paint the seven rainbow colours on these segments as shown in figure. 15.11. 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.

1. Objective Questions

1. Two plane mirrors are inclined at each other as shown in figure given figure. A ray strikes the mirror M1 at an angle 30°. The value of q is
(a) 30°

(b) 60°

(c) 45°

(d) 90°

2. The image formed by a convex mirror is

(a) erect

(b) virtual

(c) smaller in size

(d) All are correct

3. The image formed by a plane mirror is

i) real and inverted

ii) virtual and erect

iii) image distance is smaller than object distance

iv) image distance is equal to object distance

(a) only (i) and (iii) are correct

(b) only (ii) and (iv) are correct

(c) only (i) and (iv) are correct

d) only (ii) and (iii) are correct

4. The correct form of mirror equation is

(a) 1/v - 1/u = 1/f

(b)  1/v +1/f =1/u

(c)  1/v +1/u = 1/f

(d)  1/f +1/u = 1/v 

5. Pick out the correct alternative:

i) Concave mirror can form a real and inverted image.

ii) Concave mirror can form virtual and erect image.

(a) only (i) is correct

(b) only (ii) is correct

(c) both (i) and (ii) are correct

(d) Both are wrong

6. Which of the following has maximum wavelength?

(a) Blue

(b) Violet

(c) Green

(d) Red

7. Which of the following has minimum wavelength?

(a) Blue

b) Violet

(c) Green

(d) Red

8. Which of the following letter will show lateral inversion?

(a) C

(b) B

(c) N

(d) M

9. Pick out the correct alternative

i) A real image can be obtained on a screen

ii) A virtual image cannot be obtained on a screen

(a) only (i) is correct

(b) only (ii) is correct

(c) Both (i) and (ii) are correct

(d) Both are wrong

10. Rohit is observing his image in a plane mirror. The distance between the mirror and his image is 3 m. If he moves 1 m towards the mirror, then the distance between Rohit and his image will be.

(a) 2 m

(b) 3 m

(c) 4 m

(d) 8 m

Subjective Questions

1. Define reflection. Write laws of reflection.

2. Define focus, centre of curvature and radius of curvature with respect to a mirror. Draw a diagram to mark these.

3. Define real and virtual image. Give one situation each where a real and virtual image is formed.

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

5. An object is kept 10 cm away from a concave mirror of focal length 15 cm. Find the position of image.

6. An object is kept 25 cm away from a convex mirror of focal length 20 cm. Find the position of image.

7. Draw a prism and show the effect on white light when it passes through the prism. Hence define dispersion.

8. Write the characteristics of image formed by a convex mirror.

9. Write the characteristics of image formed by a concave len

10. Explain how will you make Newton’s Disc.

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