NCERT Class 8 Science Exemplar Chapter 16 - Light

Chapter 16 - Light

Multiple Choice Questions
Question 1: Part of the eye which controls the light entering is called
a) iris
b) cornea
c) lens
d) retina
Answer:
a) iris

Question 2: We can see a non-luminous object when light:
a) emitted by the object falls on the eye.
b) is reflected from the object towards our eye.
c) completely passes through the object.
d) gets completely absorbed by the object.
Answer:
b) is reflected from the object towards our eye.

Question 3: Light is falling on surface S₁, S₂, S₃ as shown in Fig.16.1.
Surfaces on which the angle of incidence is equal to the angle of reflection is/are
a) S
only
b) S
and S only
c) S
and S
d) all the three surfaces
Answer:

d) all the three surfaces

Question 4: A tiny mirror M is fixed on a piece of cardboard placed on a table. The cardboard is illuminated by light from a bulb. The position of eye with respect to position of bulb is shown in Fig.16.2 as A, B, C and D. In which position mirror will be visible?
a) A
b) B
c) C
d) D
Answer:
a) A

Question 5: A small hole P is made in a piece of cardboard. The hole is illuminated by a torch as shown in Fig. 16.3. The pencil of light coming out of the hole falls on a mirror.
At which point should the eye be placed so that the hole can be seen?
a) A
b) B
c) C
d) D
Answer:
a) A

Question 6: Two mirrors A and B are placed at right angles to each other as shown in Fig.16.4.
A ray of light incident on mirror A at an angle of 25° falls on mirror B after reflection. The angle of reflection for the ray reflected from mirror B would be
a) 25°
b) 50°
c) 65°
d) 115°
Answer:
c) 65°

Question 7: Which of the following statements is correct regarding rods and cones in the human eye?
a) Cones are sensitive to dim light.
b) Cones are sensitive to bright light.
c) Rods are sensitive to bright light.
d) Rods can sense colour.
Answer:
b) Cones are sensitive to bright light.

Question 8: In the figure of the human eye (Fig.16.5), the cornea is represented by the letter
a) A
b) B
c) C
d) D
Answer:
c) C

Very Short Answer Questions
Question 9: Name the part of the eye which gives colour to the eyes.
Answer:
The iris is the part of the eye which gives colour to the eyes.

Question 10: Boojho while waving his hand very fast in front of his eyes, observes that his fingers appear blurred. What could be the reason for it?
Answer:
The persistence of vision is the reason for the blurred vision of Boojho while waving his hand very fast in front of his eyes, observes that his fingers appear blurred.

Question 11: How many times is a ray of light reflected by two plane mirrors placed parallel and facing each other?
Answer:
Infinite number of times is a ray of light reflected by 2 plane mirrors placed parallel and facing each other.

Question 12: The angle between incident ray and reflected ray is 60°. What is the value of angle of incidence?
Answer:

Incident ray = Reflected ray
Let the incident ray and reflected ray be x.
Incident ray + Reflected ray = 60°
= x + x = 60°
= 2x = 60°
= x = 60/2
= x = 30°

Therefore, the value of angle of incidence is 30°.

Question 13: The distance between the object and its image formed by a plane mirror appears to be 24 cm. What is the distance between the mirror and the object?
Answer:

Image distance = Object distance
Let the image distance and object distance be x.
= x + x = 24 cm
= 2x = 24 cm
= x = 24/2
= x = 12 cm

Therefore, the distance between the mirror and the object is 12 cm.

Short Answer Questions
Question 14: What happens to light when it gets dispersed? Give an example.
Answer:
Light is splitted into its constituent colours, when it gets dispersed. Example: Rainbow formation is due to the dispersion of white light after passing through water droplets.

Question 15: Draw Fig.16.6 showing the position of the plane mirror. Also label the angle of incidence and angle of reflection on it.
Answer:



Question 16: Look at Fig.16.7. Can the image of the child in it be obtained on a screen?
Answer:

No, the image of the child cannot be obtained on a screen as the image is being formed in a plane mirror and the images which are formed in a plane mirror are virtual. The virtual images are those images that cannot be obtained on a screen.

Question 17: Eyes of the nocturnal birds have large cornea and a large pupil. How does this structure help them?
Answer:
A large pupil and large cornea allows more light to enter their eyes and they can see objects even in faint light.

Question 18: What kind of lens is there in our eyes? Where does it form the image of an object?
Answer:
The lens that is present in our eyes is convex lens. It focuses the light on the back of the eye, the retina. As a result, it creates an image of an object in the retina. There are sensory nerves that detect the light. There are 2 types of cells in retina namely the rods and cones. The rods are sensitive to dim light. The cones are sensitive to bright light and colour.

Question 19: Which part of the eye gets affected if someone is suffering from cataract? How is it treated?
Answer:
In old age people cannot see clearly because the eye lens becomes cloudy. This defect is called cataract. The retina of the eye gets affected if someone is suffering from cataract. It can be corrected by removing the eye lens and replacing it with an artificial lens.

Long Answer Questions
Question 20: Boojho planned an activity to observe an object A through pipes as shown in Fig. 16.8, so that he could see objects which he could not directly see.
a) How many mirrors should he use to see the objects?
b) Indicate the positions of the mirrors in the figure.
c) What must be the angle with respect to the incident light at which he should place the mirrors?
d) Indicate the direction of rays in the figure.
e) If any of the mirrors is removed, will he be able to see the objects?
Answer:

a) He should use 3 mirrors to see the object A.
b)

c) 45°
d)

e) No, he will not be able to see the objects.

Question 21: There is a mistake in each of the following ray diagrams given as Fig. 16.9 a, b, and c. Make the necessary correction (s).
Answer:

a) Direction of ray of light is wrong in this case. This should be in opposite direction such that ray should travel from incident ray to reflected ray.
b) In this case the angle of incident is not equal to the angle of reflection, this should be same.
c) The normal should be perpendicular to the surface of reflection.

Question 22: Explain the process which enables us to perceive motion in a cartoon film.
Answer:
The cartoon film we see is actually the projection of static pictures on the screen in a specific order. Usually the static pictures are shown in a sequence at the rate of 24 pictures per second one after the other giving us the perception of movement.

Question 23: How is the phenomenon of reflection used in making a kaleidoscope? What are the applications of a kaleidoscope?
Answer:
Kaleidoscope is a device that works on the phenomena of multiple reflections with the help of multiple mirrors. It has a cylindrical structure with three plane mirrors inside it. It contains several pieces of beads, broken glass, colourful bangles, pebbles etc. The observer can look into the Kaleidoscope from one end and the light enters into the Kaleidoscope from the other end. When the light falls upon these objects in the Kaleidoscope, multiple reflections of these objects are created in the three Mirrors. As a result many beautiful and colourful patterns are formed. Since the reflection of light is not always the same the observer never sees a similar pattern in the kaleidoscope.

Question 24: Fig. 16.10 shows the word REST written in two ways in front of a mirror. Show how the word would appear in the mirror.
Answer:



Question 25: Write down the names of parts of the eye in the blank spaces shown in Fig. 16.10.
Answer:

1. Ciliary muscle
2. Iris
3. Lens
4. Cornea
5. Retina
6. Optic Nerve

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