REFLECTION OF LIGHT

9. REFLECTION OF LIGHT

points to remember :

1. Transmission of light : Light coming out from a source spreads

in all directions. Light is transmitted in a straight line.

2. Reflection of light : The turning back of the rays of light after

falling on a surface is called the reflection of light.

3. Image formed in a plane mirror : The image formed in a plane

mirror is upright, laterally inverted, of the same size as the

object and as far behind the mirror as the object is in front of the

mirror.

4. Laws of reflection of light : (1) The incident ray, the reflected

ray and the normal to the reflecting surface at the point of

incidence are in the same plane. (2) The incident ray and the

reflected ray are on the opposite sides of the normal. (3) The

angle of incidence and the angle of reflection are of equal

measure.

5. Types of reflection of light : Regular reflection and irregular

reflection are the two types of reflection. If the reflecting surface

is plane and polished, the reflection is regular. If the reflecting

surface is rough, we have irregular reflection.

6. The working of a periscope and a kaleidoscope is based on the

reflection of light.

 (1) What is reflection of light ?

Ans. The turning back of the rays of light after falling on a

surface is called the reflection of light.

(2) With a neat labelled diagram, describe a simple

experiment to demonstrate reflection of light.

Ans. Fix a white paper on a drawing board and keep a plane

mirror upright on one side of it. Stick a black paper with three

narrow slits on the glass of a torch. Place the torch in front of the

mirror at about two feet from it as

shown in the figure and switch it on.

You will see three beams of light and

the bright slits of the torch in the

mirror along the lines of the reflected

beams. Light falling on the mirror is

turned back in air by the mirror. This

shows reflection of light.

* (1) Why is the candle in the mirror not called an

object ?

Ans. An object is a real body, i.e., it has physical

existence. If an object placed in front of a mirror is moved, the

image moves. If the mirror is moved, the image moves, but not

the object. That is why the candle seen in the mirror is not

called an object.

* (2) If we were to hold the candle upside down, what

would the image be like ?

Ans. If a lighted candle is held upside down in front of the

mirror, the image of the candle will be seen upside down, but

the flame will point upward (not upside down).

(3) Describe a simple experiment to show that the image

formed by a plane mirror is as far behind the mirror as the

object is in front of the mirror.

Ans. Place a plane mirror upright at one end of a chessboard

and keep a pawn on the board in the third square from the mirror.

The image of the pawn will be seen right behind the mirror in the

third square from the mirror. Now, keep the pawn in the fourth

square from the mirror. Its image will be seen right behind the

mirror in the fourth square from the mirror.

This shows that the image formed by a plane mirror is as far

behind the mirror as the object is in front of the mirror.

* (4) Where is the image formed in a plane mirror located ?

Ans. The image formed in a plane mirror is behind the mirror

and its distance from the mirror equals the distance of the object

from the mirror.

* (5) How many times is an image in a plane mirror bigger

or smaller than the object ?

Ans. The image in a plane mirror is of the same size as the

object.

* (1) What will be the nature of a person’s image in a

plane mirror as compared to the person ?

Ans. A person’s image in a plane mirror will be upright

and of the same size as the person.

* (2) Why do we use a plane mirror while getting

ready?

Ans. The image in a plane mirror is upright, of the same

size as the object and as far behind the mirror as the object is

in front of the mirror. Hence, we use a plane mirror while

getting ready.

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(6) What is lateral inversion?

Ans. If we stand in front of a plane mirror we see our image

formed in it. Our right hand appears to be the left hand in the image

and the left hand appears to be the right hand in the image. This is

called lateral inversion.

(7) State the characteristics of the image obtained in a

plane mirror.

Ans. Characteristics of the image obtained in a plane mirror :

(1) The image is upright. (2) The size of the image is equal to that of

the object. (3) The image is as far behind the mirror as the object is

in front of it. (4) The image is laterally inverted with respect to the

object.

 (9) Define the following terms : OR

Give the meaning of the following terms :

(1) Incident ray (2) Normal (3) Reflected ray (4) Angle of

incidence (5) Angle of reflection.

Ans. (1) Incident ray : A ray of light falling on a surface is

called an incident ray.

(2) Normal : The line perpendicular to the surface at the point

of incidence is called a normal.

(3) Reflected ray : The ray of light falling on a surface gets

reflected. The ray after reflection is called the reflected ray.

(4) Angle of incidence : The angle made by the incident ray

with the normal is called the angle of incidence.

(5) Angle of reflection : The angle made by the reflected ray

with the normal is called the angle of reflection.

State the laws of reflection.

Ans. Laws of reflection :

(1) The incident ray, the reflected ray and the normal to the

reflecting surface at the point of incidence are in the same plane.

(2) The incident ray and the reflected ray are on the opposite

sides of the normal.

 (3) The angle of incidence and the angle of reflection are of

equal measure.

* How should a ray of light be incident on a mirror,

placed at the right angles to the floor, in order that the

reflected ray comes downwards? Which plane will

contain these two rays and the normal?

Ans. In order that the reflected ray comes downwards,

the incident ray should be incident obliquely and downwards

on the plane mirror. The plane containing the incident ray,

the reflected ray and the normal will be perpendicular to the

paper and the mirror.

(12) What is regular reflection ?

Ans. Reflection of light from a smooth surface, such as that of a

plane mirror, is called regular reflection. In this case, if the incident

rays are parallel to each other, the reflected rays are also parallel to each

other. The angles of incidence and the angles of reflection are equal.

 (13) What is irregular reflection ?

Ans. Reflection of light from a rough surface is called irregular

reflection. In this case, even if the incident rays are parallel to each

other, the reflected rays are not parallel to each other; they are

scattered over a wider surface.

* (1) Are the laws of reflection followed when irregular

reflection takes place ?

Ans. Yes, the laws of reflection are followed when

irregular reflection takes place.

* (2) Why do rays in a parallel beam of incident light

not remain parallel to each other after reflection from a

rough surface ?

Ans. Rays in a parallel beam of incident light do not

remain parallel to each other after reflection from a rough

surface because the angle of incidence is (usually) different for

each incident ray.

* (14) How many images are obtained in two parallel

mirrors?

Ans. An infinite number of images is obtained in two parallel

mirrors.

 (16) Why are the two mirrors in a periscope placed

parallel to each other ?

Ans. The two mirrors in a periscope are placed parallel to each

other so that the rays of light emerging out of the periscope are

parallel to the rays incident on the top window of the periscope and

hence the location of the object can be seen clearly.

 (17) State the use of a periscope.

Ans. A periscope is used on a submarine to keep a watch on

things above the surface of the sea even when the submarine is

under water.

 (19) If an object is placed between two plane mirrors kept

inclined to each other at 60°, how many images will be

obtained?

Ans. If an object is placed between two plane mirrors kept

inclined to each other at 60°, five images will be obtained.

(20) How many plane mirrors are fixed in a kaleidoscope?

What is the angle between two mirrors in it ?

Ans. Three plane mirrors are fixed in a kaleidoscope and the

angle between two mirrors is 60°.

 (21) Why are many images obtained in a kaleidoscope ?

Ans. In a kaleidoscope, three plane mirrors are fixed at 60°

with each other. Hence, the rays of light starting from an object

(reflected rays) are reflected many times by the mirrors forming

many images.

(22) What is a fluorescent paint ? Where is it used ?

Ans. A paint which shines as light falls on it is called a

fluorescent paint. It is applied along the sides of a road. It can be

easily seen in reflected light and serves to guide the driver of a

vehicle when it is dark.

* Why is the surface of a glass made shiny when making

a mirror ?

Ans. The surface of a glass is made shiny when making a

mirror so that practically all the incident light is reflected by

the mirror to form a clear image.

* Q. 2 Explain the difference between regular reflection

and irregular reflection.

Ans. Regular reflection Irregular reflection

1. Regular reflection occurs

when light is incident on a

smooth surface such as that

of a plane mirror.

1. Irregular reflection occurs

when light is incident on a

rough surface.

2. In this case, if the incident

rays are parallel to each

other, the reflected rays are

also parallel to each other.

2. In this case, even if the

incident rays are parallel to

each other, the reflected rays

are not parallel to each other.

 Give scientific reasons :

* (1) The image formed in water is of the same size as the

object.

Ans. The surface of still water acts like a plane mirror. Hence,

the image formed in it is of the same size as the object.

* (2) Letters appear laterally inverted in a plane mirror.

Ans. The right and left sides are interchanged in the image

formed in a plane mirror. Therefore, letters appear laterally

inverted in a plane mirror.

* (3) Numerous images are seen in the ‘Palace of Mirrors’.

Ans. In the ‘Palace of Mirrors’, many mirrors are fixed at

various angles with each other. The image formed by one mirror

acts as an object for another mirror. Hence, numerous images are

seen in the ‘Palace of Mirrors’.

(4) You do not see the image of an object in a mirror if you

go behind the mirror to look for it.

Ans. When rays of light coming from an object, kept in front of

a mirror, fall on the mirror, they are reflected. When we stand in

front of the mirror, the reflected rays reach our eyes and we see the

image of the object. As the reflected rays appear to come from a

position behind the mirror, the image is formed behind the mirror.

But, if we go behind the mirror, the rays reflected by the mirror do

not reach our eyes and hence we do not see the image.

(5) When we approach a mirror, our image seems to

approach us at twice our speed.

Ans. In a plane mirror, the image is as far behind the mirror as

the object is in front of the mirror. As we approach the mirror, the

distance between the object and the mirror decreases. This results

in the decrease of an equal distance between the image and the

mirror. Hence, as we approach the mirror, our image seems to

approach us at twice our speed.

Q. 4 Answer the following questions in one sentence

each :

(1) State any two characteristics of an image obtained in a

plane mirror.

Ans. An image obtained in a plane mirror is upright and of the

same size as the object.

 (2) If an object, initially at 100 cm in front of a plane

mirror, is moved towards the mirror by 10 cm, what will be the

separation between the object and its image?

Ans. If an object, initially at 100 cm in front of a plane mirror,

is moved towards the mirror by 10 cm, the separation between the

object and its image will be 180 cm.

(3) If the angle of incidence is 30°, what will be the angle of

reflection ?

Ans. If the angle of incidence is 30°, the angle of reflection will

be 30°.

Q. 5 State whether the following statements are True or

False. If a statement is false, correct it and rewrite :

(1) The angle of reflection equals the angle of incidence.

(2) In an irregular reflection, the angle of reflection is not equal

to the angle of incidence.

(3) If an object is placed between two plane mirrors, kept

parallel to each other, an infinite number of images is

obtained.

Ans. (1) True. (2) False. In an irregular reflection, the angle

of reflection is equal to the angle of incidence. (3) True.

Q. 6 Fill in the blanks :

1. * (1) The incident ray, the normal and the reflected ray are all

in the same ……… .

* (2) Reflection from a rough surface is called………reflection.

* (3) In a periscope, we see the ……… reflection of an object.

* (4) In a plane mirror, the image is ……… inverted.

(5) If the angle of incidence is 60°, the angle of reflection is

……… .

Ans. (1) plane (2) irregular (3) second (4) laterally (5) 60°.

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2. (1) If two plane mirrors are placed parallel to each other

and an object is placed between them the number of

images obtained is ……… .

 (2) In a kaleidoscope, the angle between two mirrors is

……… .

(3) When light falls on a ……… paint, it shines.

(4) The plane mirrors fixed in a periscope are ……… to each

other.

(5) Reflection from a smooth surface is called………reflection.

(6) Irregular reflection occurs from a ……… surface.

Ans. (1) infinite (2) 60° (3) fluorescent (4) parallel (5) regular

(6) rough.

* Q. 7 Match the following :

‘A’ ‘B’

(1) Image in a plane mirror

(2) Regular reflection

(3) Angle of incidence_30°

(4) A shiny image

(a) Fluorescent paint

(b) Smooth surface

(c) As big as the object

(d) Angle of reflection_30°

(e) Periscope

Ans. (1) Image in a plane mirror – As big as the object

(2) Regular reflection – Smooth surface (3) Angle of incidence_

30° – Angle of reflection_30° (4) A shiny image – Fluorescent paint.