Chapter 16 : Light

MULTIPLE CHOICE QUESTIONS

1. Part of the eye which controls the light entering is called
  (a) iris    (b) cornea    (c) lens     (d) retina

Answer: (a) iris.
[ The iris controls the amount of light entering the eye by adjusting the size of the pupil. ]
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.
[ Non-luminous objects are seen when light falling on them gets reflected into our eyes.]

3.Light is falling on surface as shown in Fig.16.1.

  

                   Fig. 16.1

Surfaces on which the angle of incidence is equal to the angle of reflection is/are
 (a)  only     (b) and only     (c) and   (D) all the three surfaces

Answer: (D) all the three surfaces.

[According to the laws of reflection, the angle of incidence is always equal to the angle of reflection on all surface. ]

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: (b) B

[ According to the laws of reflection, the mirror is visible only when the angle of incidence equals the angle of reflection, directing light to the eye. ]

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.

   

                      Fig. 16.3

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

[According to the laws of reflection, the light from hole P hits the mirror and reflects at an equal angle. Tracing this path, the reflected ray points directly toward position A. ]

6.Two mirrors A and B are placed at right angles to each other as shown in Fig.16.4.

  

                     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°

[The angle of reflection at mirror A is 25°. Because mirrors are perpendicular , the angle of incidence at mirror B is 65° (= 90° – 25°) . Thus , the reflection angle is 65° .]

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.

[Cones work in bright light and detect colour, while rods are sensitive to dim light but cannot sense colour.]

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: (a)  A

[A represents the transparent cornea .The cornea is the transparent curved front part of the eye.]

VERY SHORT ANSWER QUESTIONS

9. Name the part of the eye which gives colour to the eyes.
Answer: The part of the eye that gives colour is the iris.

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 fingers appear blurred due to persistence of vision.
The image stays on the retina for about 1/16 second, so fast motion makes images overlap and appear blurred.

11. How many times is a ray of light reflected by two plane mirrors placed parallel and facing each other?

Answer: A ray of light is reflected infinite number of times between two parallel plane mirrors facing each other, due to repeated back-and-forth reflections.

12. The angle between incident ray and reflected ray is 60°. What is the value of angle of incidence?
Answer: We know that, angle of incidence is always equal to the angle of reflection .

So, 

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

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

The total distance between object and image is 24 cm, we have

The distance between the mirror and the object

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

SHORT ANSWER QUESTIONS

14. What happens to light when it gets dispersed? Give an example.
Answer: When light gets dispersed, it splits into its seven colors (VIBGYOR). For example, when sunlight passes through raindrops, it disperses to form a beautiful rainbow in the sky.

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

   

                         Fig.16.6

Answer: The plane mirror is placed horizontally at the intersection . Label the angle between incident ray and normal as x and the angle between reflected ray and normal as y .

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

         

                      Fig. 16.7

Answer: No, the image cannot be obtained on a screen because it is a virtual images cannot be projected onto screens .

17. Eyes of the nocturnal birds have large cornea and a large pupil. How does this structure help them?
Answer: Large cornea and large pupil allow more light to enter the eyes. This helps nocturnal birds see clearly in dim light or darkness, enabling them to hunt or move at night.

18. What kind of lens is there in our eyes? Where does it form the image of an object?

Answer: Our eyes have a convex lens. It forms a real and inverted image of an object on the retina. The brain later converts this inverted image into an erect one.

19. Which part of the eye gets affected if someone is suffering from cataract? How is it treated?
Answer: Cataract affects the eye lens, making it cloudy. It is treated by surgery, where the cloudy lens is removed and replaced with an artificial lens.

LONG ANSWER QUESTIONS

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.

       

                   Fig. 16.8
(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 three mirror . One mirror is needed at each of the three bends in the pipe to reflect the light .

(b) Place one mirror at each corner (bend) of the pipe. They must be positioned where the pipe changes direction to redirect the light.

(c) Each mirror must be placed at an angle of 45° to the incident light . This reflects the light at a 90° angle through the pipe .

(d) Light rays travel from object A toward the eye . The rays hit each mirror and turn 90° to follow the path of the pipe .

(e) No . If any mirror is removed, the light will hit the pipe wall and stop . Light must reflect off every mirror to reach the eye .

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).
   

                                     Fig 16.9

Answer: (a) The angles are not shown correctly.
Correction: Angle of incidence (i) must be equal to angle of reflection (r), both measured from the normal.

(b) The direction of the reflected ray is wrong.
Correction: The reflected ray should go on the opposite side of the normal, making angle r equal to angle i.

(c) The normal is not drawn properly at the point of incidence.
Correction: Draw the normal perpendicular to the surface at the point where the ray strikes, and ensure i = r.

22. Explain the process which enables us to perceive motion in a cartoon film.
Answer: We perceive motion in a cartoon film due to persistence of vision. Our eye retains an image for about 1/16th of a second. When still images change rapidly (usually 24 frames per second), the brain blends them into continuous motion.

23. How is the phenomenon of reflection used in making a kaleidoscope? What are the applications of a kaleidoscope?
Answer: A kaleidoscope uses multiple reflections of light in inclined mirrors placed inside a tube. Small pieces of coloured glass reflect many times to form beautiful, changing patterns.

Applications of a kaleidoscope:

(i) Used as a toy for entertainment.

(ii) Designers use it to create patterns for fabrics, wallpapers, and carpets.

(iii) Helps in understanding reflection and symmetry in science education.

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. Fig. 16.10

 

Answer: In the first case, with the mirror placed horizontally below, the word REST will appear upside down (vertically inverted) . In the second case, with the mirror placed vertically to the right, each letter will be flipped sideways (laterally inverted).

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

Answer: The names of parts of the eye is:

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