Class 7 Science Light Chapter Notes

Class 7 Science Light Chapter Notes. There are many more useful educational material which the students can download in pdf format and use them for studies. Study material like concept maps, important and sure shot question banks, quick to learn flash cards, flow charts, mind maps, teacher notes, important formulas, past examinations question bank, important concepts taught by teachers. Students can download these useful educational material free and use them to get better marks in examinations.  Also refer to other worksheets for the same chapter and other subjects too. Use them for better understanding of the subjects.

Light

5.1 Introduction

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

5.2 Light travels along a straight line

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

Fig. 5.1 Looking at a candle through (a) a straight pipe and (b) a bent pipe. (Illustration not reproduced.)

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 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 the incident ray with the normal is called the angle of incidence. \(\angle i\) is the angle of incidence.Angle of reflection: The angle made by the reflected ray with the normal is called the angle of reflection. \(\angle r\) is the angle of reflection.Laws of reflection

1. The incident ray, the reflected ray and the normal all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection, i.e. \(\angle i = \angle r\).

Activity 1

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.

Fig. 5.2 Image of a candle in a plane mirror. (Illustration not reproduced.)

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, the image is laterally inverted. That is, right becomes left and left becomes right.

Fig. 5.3 Lateral inversion in a plane mirror. (Illustration not reproduced.)

5.4 Types of Images

There are two types of images:

1. Real image
2. Virtual image

Real Image: The image which can be obtained on a screen is called a real image. A concave mirror forms a real image.Virtual Image: The image which cannot be obtained on a screen is called a virtual image. A plane mirror and a convex mirror form virtual images.

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.

Fig. 5.4 A concave and a convex mirror

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 the 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.

Fig. 5.5 A dentist examining a patient. Fig. 5.6 Reflector of a torch. (Illustrations not reproduced.)

Images 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.

Fig. 5.7 Convex mirror as a side view mirror. (Photograph not reproduced.)

5.6 Terms related to Spherical Mirrors

Consider a spherical mirror as shown in the given figure.

1. Centre of Curvature: The centre of the sphere of which the mirror is a part is called the centre of curvature. In the diagram, C is the centre of curvature.
2. Radius of curvature: The radius of the sphere of which the mirror is a part is called the radius of curvature. In the diagram, PC is called the radius of curvature.
3. Pole: The centre of the reflecting surface of the mirror is called the pole. In the diagram, P is called the pole of the mirror.
4. Focus: The point where all rays meet or appear to meet after reflection is called the focus. In the diagram, F is called the focus.
5. Focal length: The distance between the pole and the focus is called the focal length. In the diagram, PF is called the focal length.

5.7 Mirror Equation

The mirror equation is given by

\[ \frac{1}{v}+\frac{1}{u}=\frac{1}{f} \]

v is called the image distance, u is called the object distance and f is called the focal length.

5.8 Lenses

A 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:

1. Concave Lens: A concave lens is thinner at the middle.
2. Convex lens: A convex lens is thicker at the middle.

Fig. 5.9 (a) A convex lens and (b) a concave lens

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.

Fig. 5.10

A convex lens forms a magnified image.

A concave lens forms a 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.

Fig. 5.11 A prism splits sunlight into seven colours

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 Fig. 5.12. 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 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.

Fig. 5.12 (a) A disc with seven colours. (b) It appears white on rotating.

EXERCISES

Objective Questions

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

1. 30°
2. 60°
3. 45°
4. 90°

Answer: (b) 60°. The mirrors are at right angles. The ray strikes M₁ at 30° to the normal, so it is reflected at 30° to the normal, i.e. at 60° to the surface of M₁. Since M₂ is perpendicular to M₁, the reflected ray makes 90° − 60° = 30° with the surface of M₂, which is 90° − 30° = 60° with the normal of M₂. So θ = 60°.

The image formed by a convex mirror is

1. erect
2. virtual
3. smaller in size
4. All are correct

Answer: (d) All are correct. A convex mirror always forms an erect, virtual and smaller image.

The image formed by a plane mirror is

1. real and inverted
2. virtual and erect
3. image distance is smaller than object distance
4. image distance is equal to object distance

1. only (i) and (iii) are correct
2. only (ii) and (iv) are correct
3. only (i) and (iv) are correct
4. only (ii) and (iii) are correct

Answer: (b) only (ii) and (iv) are correct. A plane mirror forms a virtual, erect image at the same distance behind the mirror as the object is in front.

The correct form of mirror equation is

1. \( \dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f} \)
2. \( \dfrac{1}{v}+\dfrac{1}{f}=\dfrac{1}{u} \)
3. \( \dfrac{1}{v}+\dfrac{1}{u}=\dfrac{1}{f} \)
4. \( \dfrac{1}{f}+\dfrac{1}{u}=\dfrac{1}{v} \)

Answer: (c) \( \dfrac{1}{v}+\dfrac{1}{u}=\dfrac{1}{f} \)

Pick out the correct alternative:

1. Concave mirror can form a real and inverted image.
2. Concave mirror can form virtual and erect image.

1. only (i) is correct
2. only (ii) is correct
3. both (i) and (ii) are correct
4. Both are wrong

Answer: (c) both (i) and (ii) are correct. A concave mirror forms a real, inverted image when the object is beyond the focus, and a virtual, erect, magnified image when the object is between the pole and the focus.

Which of the following has maximum wavelength?

1. Blue
2. Violet
3. Green
4. Red

Answer: (d) Red. Among the colours of the rainbow, red has the longest wavelength and violet the shortest.

Which of the following has minimum wavelength?

1. Blue
2. Violet
3. Green
4. Red

Answer: (b) Violet.

Which of the following letter will show lateral inversion?

1. C
2. B
3. N
4. M

Answer: (d) M, as given in the answer key. Lateral inversion turns left into right. The letters C, B and N look different in a mirror, whereas the letter M is symmetrical, so it looks the same after lateral inversion.

Pick out the correct alternative

1. A real image can be obtained on a screen
2. A virtual image cannot be obtained on a screen

1. only (i) is correct
2. only (ii) is correct
3. Both (i) and (ii) are correct
4. Both are wrong

Answer: (c) Both (i) and (ii) are correct.

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

1. 2 m
2. 3 m
3. 4 m
4. 8 m

Answer: (c) 4 m. The image is 3 m behind the mirror, so Rohit is 3 m in front of it. After moving 1 m towards the mirror he is 2 m from the mirror, and his image is also 2 m behind it. The distance between Rohit and his image is 2 m + 2 m = 4 m.

Subjective Questions

Define reflection. Write laws of reflection.

Answer: The bouncing back of light into the same medium after striking a shiny surface is called reflection of light. Laws of reflection:

1. The incident ray, the reflected ray and the normal all lie in the same plane.
2. The angle of incidence is equal to the angle of reflection, i.e. \(\angle i = \angle r\).

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

Answer: Focus: the point where all rays meet or appear to meet after reflection is called the focus. Centre of curvature: the centre of the sphere of which the mirror is a part. Radius of curvature: the radius of the sphere of which the mirror is a part (the distance PC). The diagram marking C, F and P is given in section 5.6 above.

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

Answer: A real image is an image which can be obtained on a screen. A virtual image is an image which cannot be obtained on a screen. A concave mirror forms a real image when the object is kept far from it, for example a distant object. A plane mirror forms a virtual image, for example the image of your face in a bathroom mirror.

Write the characteristics of image formed by a plane mirror.

Answer: The image is formed behind the mirror, is virtual, erect and of the same size as the object, is at the same distance behind the mirror as the object is in front of it, and is laterally inverted (left appears as right and right as left).

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

Answer: Using the sign convention in which distances measured in the direction of the incident light are positive, for a concave mirror \(u = -10\) cm and \(f = -15\) cm. \[ \frac{1}{v}=\frac{1}{f}-\frac{1}{u}=-\frac{1}{15}+\frac{1}{10}=\frac{-2+3}{30}=\frac{1}{30} \] So \(v = +30\) cm. The image is formed 30 cm behind the mirror. It is virtual, erect and magnified (the object is between the pole and the focus).

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

Answer: For a convex mirror \(u = -25\) cm and \(f = +20\) cm. \[ \frac{1}{v}=\frac{1}{f}-\frac{1}{u}=\frac{1}{20}+\frac{1}{25}=\frac{5+4}{100}=\frac{9}{100} \] So \(v = \dfrac{100}{9}\approx +11.1\) cm. The image is formed about 11.1 cm behind the mirror. It is virtual, erect and smaller than the object.

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

Answer: When white light passes through a prism it splits into seven colours: red, orange, yellow, green, blue, indigo and violet (see the figure in section 5.9). Red bends the least and violet bends the most. The splitting of white light into its seven colours is called dispersion.

Write the characteristics of image formed by a convex mirror.

Answer: The image formed by a convex mirror is virtual, erect and smaller in size than the object. It is formed behind the mirror.

Write the characteristics of image formed by a concave lens.

Answer: A concave lens forms a virtual, erect and diminished image.

Explain how will you make Newton’s Disc.

Answer: Take a circular cardboard disc of about 10 cm diameter and divide it into seven segments. Paint the seven rainbow colours on the segments. Make a small hole at the centre and fix the disc loosely on the tip of a ball pen refill so that it can rotate freely. Rotate the disc fast in daylight. The colours mix together and the disc appears whitish.

 

 

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

EXERCISES

(c) 45°

(d) 90°

(a) erect

(b) virtual

(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 

(b) B

(c) N

(a) 2 m

(b) 3 m

Please click the link below to download Class 7 Science Light Chapter Notes.