NCERT Class 8 Science Chapter 16 - Light

Chapter 16 - Light

Question and Answers
Question 1: Define
a) Incident ray - The ray of light striking the surface is called an incident ray.

b) Reflected ray - The ray of light which is returned back into the same medium after the incident ray strikes the surface is called reflected ray.

c) Reflection - When light rays fall on an object their direction changes and they turn back. This is called reflection of light.

d) Lateral inversion - An image formed by mirror in which the left of the object appears on the right and right appears on the left. This is called lateral inversion.

Question 2: Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer: When we are in a dark room then we cannot see objects in the room. We can see the objects outside the room, as out of the room the light is available and the rays of light can enter our eyes after reflection from the objects.

Question 3: State the laws of reflection.
Answer: The laws of reflection include
• The angle of incidence is equal to angle of reflection.
• The incident ray, reflected ray and the normal all lie in the same plane.

Question 4: Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.
Answer:
→ Aim:
To show that the incident ray, reflected ray and the normal at the point of incidence lie in the same plane.

→ Materials Required:
Plane mirror, holder, ray box, etc.

→ Procedure:
Fix sheet of white paper, a little beyond the edge of the board. Place a plane mirror strip vertically to the paper using a stand. Throw light from a ray box on the mirror. Look at the reflected ray. Mark the incident ray, normal ray and reflected ray. Fold the paper which is beyond the edge of the board. You will observe that the reflected ray is not seen in the folded portion of the chart paper. Now bring the folded portion back to its original position. The reflected ray of light is again seen on the page.

→ Conclusion:
The sheet on the board can be considered as a plane. The incident ray, the reflected ray, the normal at the point of incidence lie in the same plane. Incident ray, reflected ray and normal at the point of incidence lie in the same plane.

Question 5: Define

a) Regular Reflection - When parallel rays of light fall on a smooth surface, they are reflected back as parallel rays only. This is called regular reflection.

b) Irregular Reflection - When parallel rays of light fall upon an irregular surface, they are reflected back into all the direction. This is called irregular reflection or diffused reflection.

Question 6: Differentiate between regular and diffused reflection. Does the diffused reflection mean the failure of laws of reflection.

Answer:
Regular reflection:

• All the reflected are parallel.

• It occurs on a smooth and polished surface.

• Reflected rays are in one direction.

Irregular or Diffused reflection:

• The reflected rays are not parallel. 

• It occurs on the rough surface. 

• Reflected rays are scattered in different directions.

No, the diffused reflection doesn't mean the failure of laws of reflection. It is because of the irregularities in the surface, reflected rays scatter in different directions.

Question 7: Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

(a) Polished wooden table.

(b) Chalk Powder

(c) Cardboard surface

(d) Marble floor with water spread over it

(e) Mirror

(f) Piece of paper

Answer:

a) Regular reflection will take place as the surface is plane and polished.

b) Diffused reflection will take place as the surface is rough.

c) Diffused reflection will take place as the surface is rough.

d) Regular reflection will take place as the surface is smooth and plane.

e) Regular reflection will take place as the surface is plane and polished.

f) Diffused reflection will take place as the surface is rough.

Question 8: What is the angle of reflection of a ray if the reflected ray at an angle of 90° of the incident ray?

Answer:

Given, the angle of reflection is 90°. According to the laws of reflection, angle of incidence is equal to angle of reflection. 

The angle between the incident ray and reflected ray is 90°.

= ∠i + ∠r = 90°
= ∠i + ∠i = 90° (as ∠i = ∠r)
= 2∠i = 90°
= ∠i = 45°

Therefore, the angle of incidence is 45°.

Question 9: Define:
a) Dispersion of Light - The splitting up of light into its constituent colors while passing from one medium to the other is called dispersion of light.

b) Blind spot - There is a point in the eye located at the junction of the retina and the optic nerve where no sensory cells are present. This spot is therefore called the blind spot as it does not support any vision.

Question 10: Describe the construction of Kaleidoscope.
Answer: Kaleidoscope is a device based on the principle of multiple reflections. It consists of three long and narrow strips of plane mirrors inclined at an angle of 60° to one another forming prism. This is fitted in a tube. One end of this tube is closed by a cardboard disc having a hole at its centre. To the other end touching the mirrors plane glass plate is fixed on which broken pieces of coloured bangles are placed. This end of the tube is closed by a ground glass plate.

Question 11: Draw a labelled sketch of human eye.
Answer:

Question 12: How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Answer: Infinite images are formed when two plane mirrors are placed parallel to each other, irrespective of the distance.

Question 13: Define:
a) Cornea - It is a transparent covering present on the outer side of the eyes. It consists of 6 layers. Tear fluid covers cornea and protects the eye.

b) Braille - Braille system is a system used by visually challenged persons (blind persons) to read and write. It has 63 dot patterns which represents alphabets, common words or signs.

Question 14: 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 advise?
Answer: Teacher has advised Gurmit not to do so because laser light is very harmful for her eyes and can cause a permanent defect in the eye. Person can even lose his or her eyesight if laser torch is directed over the eyes.

Question 15: Explain how you can take care of your eyes.
Answer: The following are the ways that will help a person to take care of his/her eyes:
• If there is any problem of the eyes have a checkup and use suitable spectacles if advised.
• Too little or too much light is bad for eyes.
• Don't look at the sun or very bright light directly.
• If dust particles enter the eye don't rub the eyes. Wash your eyes with clean water.
• Always read at a normal distance. Don't read the keeping the book to close to the eyes or keeping it too far.
• Defiency of Vitamin A causes nightblindness. So take food rich in Vitamin A.

Question 16: 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.

Answer:

Question 17: Boojho stands at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see himself in the mirror? Also, can he see the image of objects situated at P, Q and R?

Answer: No, Boojho can’t see himself in the mirror. He can see the image of the object at P and Q but not of R.

Question 18:
a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
b) Can Paheli at B see this image?
c) Can Boojho at C see this image?
d) When Paheli moves from B to C, where does the image of A move?

Answer:
a)

b) Yes, Paheli can see the image of A.
c) Yes, Boojho can see the image of A.
d) Image of the object at A will not move as an object is not moving.