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.

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.