CHAPTER – 4 LIGHT ENERGY [EXERCISE SOLUTIONS]

A. Choose the correct option : (d) blue, green and red

1. The angle of incidence on a plane mirror is Answer : (d) blue, green and red
300. The angle of reflection will be :
(a) 300 (b) 600 B. Write T for true and F for False
(c) 15 0
(d) 00 statements :
Answer : (a) 300 1. The bouncing back of light into its original
medium after striking a surface is called
2. Which of the following will produce refraction of light.
diffuse reflection of light? Answer : False.
A. Plane mirror B. Piece of paper
C. Still water in lake D. Leather bag 2. When a parallel beam of light strikes a
(a) A and B (b) B and C rough surface, the light rays reflects in
(c) A and D (d) B and D different directions producing diffused
Answer : (d) B and D reflection.
Answer : True.
3. If you stand 1 m in front of a plane mirror,
how far away would you see yourself in the 3. The angle of incidence is always equal to
mirror? angle of reflection.
(a) 1 m (b) 2 m Answer : True.
(c) 3 m (d) 4 m
Answer : (b) 2 m 4. The image of the right hand in a plane
mirror looks like left hand.
4. The images formed on the plane mirror is : Answer : True.
(a) real (b) virtual
(c) diverging (d) converging 5. A virtual image may be inverted or erect.
Answer : (b) Virtual Answer : False.

5. Which one of the following is the best 6. The image formed by a plane mirror is a
reflector of light? real image.
(a) Plastic plate (b) Plane mirror Answer : False.
(c) Wall (d) Paper
Answer : (b) Plane mirror 7. In irregular reflections, we can see objects
clearly.
6. The plane mirror forms a : Answer : False.
(a) virtual image (b) real image
(c) inverted image (d) magnified image 8. The image formed by a plane mirror is
Answer : (a) virtual image diminished.
Answer : False.
7. The interchange of left and right between
an object and its image is called : 9. A ray of light obtained as a result of
(a) lateral inversion (b) refraction reflection from a surface is called an incident
(c) reflection (d) scattering ray.
Answer : (a) lateral inversion Answer : False.

8. The primary colours are : 10. In reflection, the angle of incidence is

(a) red, blue and yellow always greater than the angle of reflection.
(b) magneta, yellow and cyan Answer : False.
(c) red, blue and red
11. The image formed by a plane mirror is of on a reflecting
the same size as the object. surface
Answer : True. 3. Normal (c) Angle between
the normal and the
C. Fill in the blanks : incidence ray
1. A black surface reflects 4. Reflected ray (d) The point where
almost……………light. the incident ray
Answer : no strikes
5. Angle of incidence (e) Angle between
2. The polished, shiny and smooth surfaces the normal and
reflect light in a…………….way. reflected rays
Answer : regular 6. Angle of reflection (f) A polished surface
which can reflect
3. A narrow beam of light is called a…………… back the rays of light
Answer : ray 7. Reflecting surface (g) Ray of light that
returns back
4. The perpendicular to the surface at the
Answer : 1 – (b) A light ray falling on a
point of incidence is called the……………. reflecting surface
Answer : Normal. 2 – (d) The point where the incident ray
strikes
5. ……………mirrors are used in making 3 – (a) A line drawn perpendicular to the
periscopes. reflecting surface at the point of incidence
Answer : Plane 4 – (g) Ray of light that returns back
5 – (c) Angle between the normal and the
6. The……………image is just an illusion.
incidence ray
Answer : virtual
6 – (e) Angle between the normal and
reflected rays
7. The retina of the human eye has…………..
7 – (f) A polished surface which can reflect
receptors for coloured light. back the rays of light
Answer : photoreceptors
E. Answer the following questions in
8. Any three colours that produce white light short.
when combined with the correct intensity 1. What is reflection?
are called………………colours of light. Answer : The process of sending back the light
Answer : primary rays which fall on the surface of an object, is
called reflection of light.
9. The production of various colours of light
by the mixing of the three primary colours of 2. What are the uses of plane mirror?
light is known as…………… Answer : (i) Plane mirrors are used at home to
Answer : colour addition view ourselves.
(ii) Plane mirrors are used in making
10. A rainbow is always formed in the periscopes.
direction opposite to the position of (iii) Plane mirrors are fixed on the walls of
the……………. certain shops (like jewellery shops) to make
Answer : sun the shops look bigger.
D. Match the following :
3. State the difference between real and
1. Incident ray (a) A line drawn
virtual images.
perpendicular to the
Answer : Difference between virtual image
reflecting surface at
and real image are :
the point of incidence
2. Point of incidence (b) A light ray falling
 Real Image Virtual Image F. Answer the following questions in
1. A real image is A virtual image is formed detail.
formed by actual when reflected rays 1. Draw a diagram and explain the laws of
intersecting of the appear to meet when reflection.
reflected light rays. produced in backdward Answer : When a ray of light is reflected from
direction. a surface, the reflected ray obeys certains
2. It can be obtained on It cannot be obtained on laws. These are called the laws of reflection.
screen. screen.
3. A real image is A virtual image is always
always inverted. erect.
4. It is formed on the It is formed on the
same side of the backside of the mirror.
mirror.

4. State the characteristics of the image

(i) First law of reflection : The angle of
formed by a plane mirror.
incidence is always equal to the angle of
Answer : (i) The image formed in a plane
reflection.
mirror is virtual (or unreal). It cannot be
Angle of incidence = Angle of reflection
obtained on a screen.
(ii) The image formed in a plane mirror is at
(ii) Second law of reflection : The incidence
the same distance behind the mirror as the ray, the reflected ray and the normal to the
object is in front of it. mirror at the point of incidence, all lie in the
(iii) The image in a plane mirror is of the same same plane.
size as the object. It is neither enlarged nor
diminished.
2. Explain the characteristics of the image
(iv) The image in a plane mirror is erect. It is
formed by a plane mirror with the help of an
the same side up as the object.
example.
(v) The image in a plane mirror is laterally Answer : Characteristics of the image formed
inverted (sideways reversed) with respect to
by a plane mirror :
the object.
(i) The image formed in a plane mirror is
virtual (or unreal). It cannot be obtained on a
5. Define dispersion. screen.
Answer : Dispersion is defined as the splitting (ii) The image formed in a plane mirror is at
of light in different colours.
the same distance behind the mirror as the
object is in front of it.
6. What are primary colours? Name them.
(iii) The image in a plane mirror is of the same
Answer : Any three colours that produce
size as the object. It is neither enlarged nor
white light when combined with the correct diminished.
intensity are called primary colours of light. (iv) The image in a plane mirror is erect. It is
the same side up as the object.
7. What is a pure pigment? Which colour’s
(v) The image in a plane mirror is laterally
light do the green pigments absorb? inverted (sideways reversed) with respect to
Answer : A pigment that absorbs a single the object.
frequency or colour of light is known as a pure
pigment.
Green pigments absorb Magneta light.

8. What is a rainbow?
Answer : A band of colours in the sky
extending from violet to red is called a
rainbow.
3. Explain the importance of colour
subtraction with an example.
Answer : The process of color subtraction is a
useful means of predicting the
ultimate color appearance of an object if
the color of the incident light and the
pigments are known. By using the
complementary color scheme, the colors of
light that will be absorbed by a given material
can be determined.
These colours are subtracted from the
incident light colours (if present) and the
colours of reflected light (or transmitted light)
can be determined. Then the color
appearance of the object can be predicted.

4. Explain the formation of rainbow.

Answer : A rainbow is formed by the
refraction, reflection and dispersion of the
sunrays through raindrops. A rainbow is a
natural spectrum which appears in the sky
after rain. The sunrays are dispersed by the
water droplets present in the atmosphere.
The water droplets act as small prisms which
refract and disperse the sunrays falling on
them. The sunrays emerging out of the water
droplets are refracted again. A rainbow is
always formed in the direction opposite to the
position of the sun.