Light Energy

(a) False
Correct Statement — The image formed by a plane mirror is virtual.

(b) False
Correct Statement — When a light ray is reflected from a wall, the angle of incidence is equal to the angle of reflection.

(c) True

(d) True

(e) False
Correct Statement — The image formed by a plane mirror cannot be obtained on a screen.

(f) True

(g) True

(h) False A rose appears red in white light. Correct Statement —

(i) True

(j) True

(a) Angle of reflection

(b) in one plane

(c) the object is in front of it

(d) erect, virtual

(e) irregular

(f) virtual

(g) silvering

(h) does not

(i) reflects

(j) secondary

3 m

Reason — The distance of image from the mirror = The distance of object from the mirror. Since the distance between the man and image is 6 m, so the distance between mirror and man is 3 m.

35°

Reason — The angle of incidence = angle of reflection. Since angle between incident and reflected ray is 70°, so angle of incidence is 35°.

virtual and of same size

Reason — The image formed by a plane mirror is virtual and of same size as that of object.

30°

Reason — The angle of incidence = angle of reflection.

60°

Reason — The angle between the incident ray and the reflected ray = angle of incidence + angle of reflection, and the angle of incidence = angle of reflection.

reflection

Reason — The property due to which a light ray striking a surface is returned back into the same medium is called reflection.

reflected ray

Reason — A ray of light after reflection from a mirror is known as reflected ray.

air

Reason — The speed of light is maximum in air i.e. 3 x 10⁸ m s^-1.

black

Reason — A red rose absorbs the green light falling on it and reflects none, so it appears black.

Blue, green and red

Reason — Red, green and blue are the three primary colours.

The returning of light in the same medium after striking a surface is called reflection of light.

A plane mirror is made by silvering one side of a thin plane glass plate. The silvered surface is then coated with some opaque material so as to safeguard the silvering on it.

1. The ray of light that strikes the surface is called the incident ray.

2. The ray of light which is returned back into the same medium after striking the reflecting surface is called the reflected ray.

3. The angle between the incident ray and the normal is called the angle of incidence. It is represented by the letter i.

4. The angle between the reflected ray and the normal is called the angle of reflection. It is represented by the letter r.

5. The perpendicular drawn on the surface at the point of incidence is called the normal.

Below is the labelled diagram showing the reflection of a ray of light by a plane mirror:

The two laws of reflection of light are:

1. The angle of incidence is equal to angle of reflection.
2. The incident ray, the reflected ray and the normal all lie in the same plane.

The laws of reflection of light can be verified by the following experiment:

1. Take a drawing board and a white sheet of paper. Fix the white sheet of paper on the drawing board. Draw a straight line MM₁ on the paper.
2. On the line MM₁ take a point O at its middle and draw a line OA such that ∠MOA is less than 90°. Draw a normal ON on MM₁ at point O.
3. Take a small plane mirror and place along the line MM₁. Use plasticine to fix the mirror on the paper so that it stands upright.
4. Fix pins P and Q vertically on the line OA. Set your eye on the other side of the normal ON along the direction as shown in below figure.
5. You will see images P^', Q^' of pins P and Q in the mirror.
6. Set your eye in line with the two images P^', Q^' and fix two more pins R and S on the paper in front of the mirror such that both the pins R and S are in line with P^', Q^'.

7. Remove the mirror and draw small circles around pins P, Q, R and S. Draw a line OB joining the points R and S.
8. AO is incident ray, OB is reflected ray and ON is normal at the point of incidence O.
9. Using a protractor measure angle of incidence i (∠AON), angle of reflection r (∠BON). We will find two angles are equal.

This experiment verifies the two laws of reflection:

1. Angle of incidence = Angle of reflection.
2. Incident ray, reflected ray and normal all lie in the same plane of paper which is the second law of reflection.

The angle of incidence is 0°.

Below diagram shows the reflection of a light ray incident normally on a plane mirror:

(a) The angle of incidence is 15°.

(b) The angle of reflection is 15°.

(a) Angle of incidence is 0°.

(b) Angle of incidence is 30°.

(c) Angle of incidence is 45°.

Below diagram shows the reflected ray:

Law used to draw the direction of the reflected ray is:

The angle of incidence is always equal to the angle of reflection.

Normal ON is perpendicular on plane mirror, so

Angle of incidence = 90° - 30° = 60°.

Angle of reflection = 60°. (Since angle of incidence = angle of reflection)

For an object in front of a plane mirror, the image is formed behind the mirror. The image is formed when the reflected rays are produced backwards. The image is as far behind the mirror as the object is in front of it.

Below ray diagram shows the formation of image of a point object by a plane mirror:

The ray diagram showing formation of the image of point object O placed in front of a plane mirror is given below:

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

1. The image is upright or erect.
2. The image is virtual.
3. The image is of same size as the object.
4. The image is laterally inverted.

The image formed is as far behind the mirror as the object is in front of it.

(a) The distance of the image from the mirror is 2 metre as the image formed is as far behind the mirror as the object is in front of it.

(b) The distance between object and image will be 4 m (object distance + image distance = 2 m + 2 m)

The interchange of the left and right sides in the image of an object in a plane mirror is called lateral inversion.
Below diagram illustrates lateral inversion of an image:

Letter C and I will appear as shown below, when seen in a plane mirror:

When a beam of light falls on a surface which is not perfectly smooth and polished such as a wall, wood, paper etc, the different portions of the surface reflect light in different directions. Such a reflection of light from an uneven surface is called irregular or diffused reflection. Reflection of light from a wall is an example of irregular reflection.

We are able to see the objects around us because of the diffused reflection of light from them in all directions.

The two uses of a plane mirror are:

1. It is used as a looking glass.
2. It is used in barber's shop for seeing the rear view at the back.

Yes, light can travel in vacuum.

(a) The speed of light in air is 3 x 10⁸ m s^-1.

(b) The speed of light in glass is 2 x 10⁸ m s^-1.

(a) When light goes from air to glass, it slows down.

(b) When light goes from glass to water, it speeds up.

(c) When light goes from water to air, it speeds up.

Primary colours are the colours of light by mixing which white light is obtained but they themselves cannot be obtained by mixing any other colours of white light. The three primary colours are red, green and blue.

Secondary colours are those colours which are obtained by mixing any of the two primary colours. Yellow, cyan and magenta are the three secondary colours.

(a) Green

(b) Green

(c) Magenta

(d) Yellow

Leaves appear green in white light because they reflect only the green light and absorb the light of all other colours.

(i) A red rose appears black in green light because it absorbs the green light falling on it and reflects none.

(ii) A red rose appears bright red in red light because it reflects the red light falling on it and absorbs none of it.

A piece of paper appears white in sunlight because it reflects light of all the colours. When the paper is viewed in red light it appears red colour because it reflects the red light falling on it.

When seen in red light, the piece of paper will appear black only. This is because the piece of paper absorbs light of all colours therefore it appears black.