Class 8 Science

Light

NCERT Solution 2

Question 9: Describe the construction of a kaleidoscope.

Answer: A kaleidoscope is composed of a hollow tube. Three mirrors are placed in the form of a triangular tube and their reflecting surfaces face each other. One end of the tube is covered with a transparent sheet and another end is covered with an opaque sheet. There is an eyehole in the opaque sheet. Bits of glass are filled inside the tube. When a kaleidoscope is turned, we get to see various patterns in it. These patterns are formed because of multiple reflections.

Question 10: Draw a labeled sketch of the human eye.

Answer:

structure of human eye

Question 11: Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advice?

Answer: Laser rays can be very harmful for the eyes. Hence, a laser torch should never be used for activities related to light.


Question 12: Explain how you can take care of your eyes.

Answer: Some tips for care of eyes are as follows:

  • Do not read in too bright or too dim light.
  • Do not look directly at a bright object or at the sun.
  • Do not keep the book too close to your eyes; while reading. Don’t keep the book too far either.
  • If something gets into the eye, do not rub the eye. Wash it with cold water.
  • In case of any problem; like itching or burning sensation; consult an ophthalmologist. A doctor who specializes in the disease of eyes is called an ophthalmologist.

Question 13: What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Answer: We know that the angle of incidence is equal to the angle of reflection.

Given: ∠ i + ∠ r = 90°
Since ∠ i = ∠ r hence;
∠ i + ∠ i = 90°
Or, 2∠ i = 90°
Or, ∠ i = 45°


Question 14: How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Answer: When the mirrors are parallel to each other, then we get infinite number of images.

Question 15: Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 16.19. Draw the reflected ray from the second mirror.

figure for question 15

Answer:

answer figure for question 15

Question 16: Boojho stands at A just on the side of a plane mirror as shown in the given figure. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?

figure for question 16

Answer: Boojho cannot see his image in the mirror. But he can see P, Q and R. This can be shown by the following figure.

answer figure for question 16

Question 17: Find out the position of the image of an object situated at A in the plane mirror (given figure).

figure for question 17
  • Can Paheli at B see this image?
  • Can Boojho at C see this image?
  • When Paheli moves from B to C, where does the image of A move?

Answer: Both Paheli (at B) and Boojho (at C) can see the image. The position of the image will not change with change in position of the viewer, because the object remains at the same position. Image position can be drawn as in the previous question.