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CNMS ICSE - 7 Std
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Answers to textbook exercises

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Chapter: 04. Light

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Check Your Progress (Page 69)

The bouncing back of light from a surface is called
a. reflection of light.
The distance of an object from the mirror is
b. equal to the distance of its image from the mirror.
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My Question Time (Page 69)

Frame a question on each of the following topics and discuss their answers.
a. reflection of light
Question: What is meant by the reflection of light? ​Answer: The bouncing back of light rays into the same medium after striking a surface is known as the reflection of light.
b. plane mirrors
Question: What are three major characteristics of the image formed by a plane mirror? ​Answer: The image is of the same size as the object, is laterally inverted, and its distance behind the mirror is equal to the distance of the object in front of the mirror.
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Reason Corner (Page 69)

A plane mirror is inclined at 45° to the floor. An incident ray parallel to the floor strikes the mirror and forms a reflected ray. What is the angle of reflection?
45° (Since the mirror is inclined at 45° to the floor and the incident ray is parallel to the floor, the angle between the incident ray and the mirror surface is 45°. Therefore, the angle of incidence with the normal is 90° - 45° = 45°. By the law of reflection, the angle of reflection is also 45°).
A doll is kept at a distance of 3 m from a plane mirror. What is the distance between the doll and its image?
6 m (The distance of the doll from the mirror is 3 m, and the distance of its image behind the mirror is also 3 m. Therefore, the total distance between the doll and its image is 3 m + 3 m = 6 m).
Millie wrote her name on a board and held it up in front of a plane mirror. What did her name look like in the mirror?
b. 31JJIM
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Check Your Progress (Page 75)

Speed of light in water is
a. 2.25 × 10⁸ m/s.
Which of the following is not a colour of the rainbow?
c. brown
Which of the following is a primary colour?
c. blue
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My Question Time (Page 75)

Frame a question on each of the following topics and discuss their answers.
a. speed of light
Question: What is the approximate speed of light in a vacuum or air? ​Answer: The speed of light in a vacuum or air is approximately 3 × 10⁸ m/s.
b. colour subtraction
Question: What is colour subtraction? ​Answer: Colour subtraction is the process of determining the observed colour of an object by subtracting (absorbing) specific colours of light from the incident light thrown on it.
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Reason Corner (Page 75)

What will be the colour of the following objects?
a red rose in green light
Black (The red rose absorbs the green light and reflects nothing).
a red rose in red light
Red (The red rose reflects the red light).
a white paper in red light
Red (The white paper reflects all wavelengths; since only red light is thrown on it, it reflects red light).
a black paper in red light
Black (The black paper absorbs all colours of light, including red, reflecting nothing).
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Prepping for PISA (Page 76)

Assertion (A): A closed room gets light even when no direct sunlight falls inside the room. Reason ®: In diffused reflection, incident rays are reflected at different angles and in different directions.
a. Both A and R are true and R is the correct explanation of A.
Mark the correct statements about diffused reflection.
a. Diffused reflection is caused due to irregularities in the reflecting surface.
c. Diffused reflection follows laws of reflection.
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Virtual Tour (Page 77)

What are the materials required for the experiment?
A drawing board, a white sheet of paper, board pins, a plane mirror, a plane mirror holder, a protractor, a pencil, and a ruler.
Write down the steps of the experiment.
Fix the white sheet of paper on the drawing board using board pins.
Draw a line XY to mark the position of the mirror and place the plane mirror on XY using a mirror holder.
Fix two board pins vertically at points P and Q in front of the mirror.
Place two more pins at points R and S such that R, S, and the images of P and Q (P’ and Q’) all lie in a straight line.
Mark these points with a pencil, remove the pins, and draw the incident ray AO (passing through P and Q), the reflected ray BO (passing through R and S), and the normal ON perpendicular to XY at the point of incidence O.
Measure the angles of incidence (∠AON) and reflection (∠BON) using a protractor.
What is the observation and conclusion?
Observation: The angle of incidence (∠AON) is equal to the angle of reflection (∠BON). Additionally, the incident ray, reflected ray, and normal all lie on the same sheet of paper (same plane).
Conclusion: The laws of reflection are verified.
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Evaluation (Page 78)

A. Choose the correct option.
Reflection of light occurs
c. when light is bounced back by the object.
White light is composed of
a. seven colours.
If a ray of light is incident perpendicularly on a plane mirror, the angle of incidence will be
d. 0°.
If an object is placed at a distance of 0.25 m in front of a plane mirror, the distance between the object and the image formed by the mirror will be:
d. 0.5 m
An incident ray A strikes a plane mirror as shown in the ray diagram given. Identify the reflected ray.
b. C (as the angle of reflection must equal the angle of incidence relative to the normal B).
B. Fill in the blanks.
The ray of light falling on a reflecting surface is called the _________________________.
incident ray
The image formed by a plane mirror is _________________________ inverted.
laterally
A _________________________ is an optical instrument used in submarines to see objects above the surface of water.
periscope
Speed of light slows down when it travels through _________________________ materials.
transparent
The change in the direction of light by a mirror is called _________________________ of light.
reflection
C. Write true (T) or false (F) against the following statements.
According to the law of reflection, the angle of incidence is always equal to the angle of reflection.
T
In case of a plane mirror, the size of the image is different from the size of the object.
F
The three primary colours are red, green and yellow.
F
If we subtract red from yellow, we get green.
T
D. Name the following.
After reflection, the right side of an object appears to be on the left side in its image.
Lateral inversion
An actual representation of an object.
Image
An instrument used by submarines to observe objects above the surface of water.
Periscope
The colours produced by mixing any two primary colours of light.
Secondary colours (or Composite colours)
E. Differentiate between the following.
incident ray and reflected ray
Incident ray: The ray of light that travels towards and strikes a reflecting surface.
Reflected ray: The ray of light that bounces back into the same medium after striking the reflecting surface.
angle of incidence and angle of reflection
Angle of incidence: The angle formed between the incident ray and the normal at the point of incidence.
Angle of reflection: The angle formed between the reflected ray and the normal at the point of incidence.
primary colours and secondary colours
Primary colours: The basic colours of light (red, green, and blue) that cannot be created by mixing other colours.
Secondary colours: The composite colours (magenta, cyan, and yellow) produced by mixing any two primary colours of light.
F. Short-answer-type questions.
What are the two types of mirrors?
The two types are plane mirrors and spherical mirrors.
What are the three characteristics of an image formed by a plane mirror?
The size of the image is the same as the size of the object.
The image formed is laterally inverted.
The distance between the image and the mirror is the same as the distance between the object and the mirror.
State the two laws of reflection.
The incident ray, the reflected ray, and the normal at the point of incidence always lie in the same plane.
The angle of incidence and the angle of reflection are always equal.
What are primary colours? Which colour is made when they are mixed together?
Primary colours are colours of light (red, green, and blue) that cannot be obtained by mixing other colours. When all three are mixed together, they produce white light.
Which two factors are majorly responsible for the colour of an object?
The specific colour that the object reflects or absorbs.
The colour present in the light that is thrown on the object.
Which two colours are mixed to make the colour
a. yellow?
Red and Green b. cyan?
Blue and Green c. magenta?
Red and Blue
Name the seven colours of a rainbow.
Violet, Indigo, Blue, Green, Yellow, Orange, and Red.
G. Long-answer-type questions.
Write five uses of a plane mirror.
They are used as makeup mirrors or looking glasses.
They are used in barber shops to see the back of the head.
They are kept on opposite walls of a room to make it appear more spacious.
They are used in solar cookers and solar geysers to reflect sunlight.
They are used to build optical instruments like periscopes and kaleidoscopes.
Verify the two laws of reflection with the help of an activity.
Aim: To verify the laws of reflection.
Materials required: A drawing board, white paper, board pins, plane mirror, mirror holder, protractor, pencil, and a ruler.
Procedure: Fix the white paper on the drawing board. Draw a straight line XY representing the mirror’s position and place the plane mirror on it. Fix two pins vertically at P and Q in front of the mirror. From the opposite side, look at the images of P and Q (P’ and Q’) and place two more pins R and S such that R, S, P’, and Q’ all form a straight line. Trace the lines AO (through P and Q) representing the incident ray, and BO (through R and S) representing the reflected ray. Draw the normal ON perpendicular to XY at point O.
Observation: Using a protractor, measure ∠AON (angle of incidence, i) and ∠BON (angle of reflection, r). We observe that ∠AON = ∠BON. Additionally, all rays and the normal lie on the flat sheet of paper (same plane).
Conclusion: The laws of reflection are verified.
Explain colour subtraction with the help of an example.
Colour subtraction is the process of finding the resulting colour of an object by subtracting (absorbing) certain colours of light from the incident light.
Example: Consider white light (which is a mixture of red, green, and blue light) thrown on a shirt that absorbs red light. The red colour is subtracted from the white light: ​
math
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Thus, the shirt reflects green and blue light, making it appear cyan to our eyes.
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Higher Order Thinking Skills (HOTS)

A ray of light strikes a reflective plane at an angle of P = 48° with the plane surface as shown in the figure. Find the following. a. ∠i
42° (Since the normal is perpendicular to the surface, the angle of incidence ∠i = 90° - 48° = 42°).
b. ∠r
42° (Since the angle of reflection is always equal to the angle of incidence, ∠r = ∠i = 42°).
c. ∠Q
48° (The angle between the reflected ray and the plane surface is 90° - ∠r = 90° - 42° = 48°).
If you see a green leaf on a tree, it indicates that
d. Green colour is reflected off the leaf.
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Integrate (Page 77)

Newton’s disc: Find out the reason for the observation.
Reason: When the Newton’s disc is rotated at a high speed, the individual colours cannot be distinguished by the human eye due to the persistence of vision. The brain fuses the rapidly changing colours together, causing the rotating disc to appear white.
What are the materials used to make mirrors? How were mirrors made in ancient times? Who invented the first modern mirror?
Materials: Glass coated with a thin reflecting layer of metal (such as aluminum or silver) and protected by a backing of protective paint.
Ancient mirrors: Ancient mirrors were typically made of highly polished stones (like obsidian) or polished metals such as bronze, copper, silver, or gold.
First modern mirror: The German chemist Justus von Liebig invented the silvered-glass mirror in 1835 by applying a thin layer of metallic silver to glass through chemical reduction.

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