Light Chapter 12 Class 8 Question Answers

“Light” is Chapter 12 taken from the JKBOSE textbook of Science for students of Class 8th. In the previous post, I provided you with Questions and Answers for Chapter 11 Materials Metals and Non Metals. This post is about the Light Chapter 12 Class 8 Question Answers. Before we move on to solutions you must have your basics clear. So, let me begin by giving you a brief overview of what you have studied in this chapter.

Light Chapter 12 Class 8 Question Answers

Overview of Chapter

• WHAT MAKES THINGS VISIBLE?
• LAWS OF REFLECTION.
• REGULAR AND DIFFUSED REFLECTION.
• REFLECTED LIGHT CAN BE REFLECTED AGAIN.
• MULTIPLE IMAGES.
• SUNLIGHT- WHITE OR COLOURED.
• WHAT IS INSIDE OUR EYES?
• CARE OF EYES.
• VISUALLY CHALLENGED PERSONS CAN READ AND WRITE.
• WHAT IS A BRAILLE SYSTEM?

WHAT MAKES THINGS VISIBLE: The presence of light makes things visible. We can see things from which light comes or which it is reflected.

LAWS OF REFLECTION: There are two laws of reflection:

1. First Law of Reflection: This law states that incident ray, reflected ray and normal all lie in the same plane.
2. Second Law of Reflection: This law states that the angle of incidence is always equal to the angle of reflection. ∠i = ∠

REGULAR AND DIFFUSED REFLECTION: In regular reflection, a parallel beam of incident light is reflected as parallel beam in one direction. Regular reflection takes place from smooth surfaces like plane mirrors and highly polished metal surfaces.

In diffused reflection, a parallel beam of incident light is reflected in different directions. The diffused reflection takes place from rough surfaces like paper, cardboard, walls, and unpolished metal objects.

REFLECTED LIGHT CAN BE REFLECTED AGAIN: When light rays reflected from a plane mirror are incident on another plane mirror, the light rays are reflected again. An instrument in which reflected light is reflected again is called a periscope.

MULTIPLE IMAGES: When two plane mirrors are inclined at an angle, they can form multiple images of an object.

SUNLIGHT- WHITE OR COLOURED: Sunlight is termed white light but it consists of seven colours. The white light from the sun can be split into seven different colours by passing it through a glass prism. The process is termed as dispersion of light.

WHAT IS INSIDE OUR EYES? The eye enables us to see various objects around us. The main parts of the human eye are Cornea, Iris, Pupil, Eye lens, Ciliary muscles, Retina and Optic nerve.

CARE OF EYES: Some important preventive measures to protect our eyes.

1. Wash your eyes twice a day.
2. Do not keep the book too close to your eyes while reading.
3. Too little light or too much light is harmful to the eyes.
4. Consult an eye specialist in case of an injury.
5. Do not rub your eyes if something enters your eye. Wash it with water.
6. Include those foods which contain vitamin A in your diet.

VISUALLY CHALLENGED PERSONS CAN READ AND WRITE: Visually challenged persons try to identify things by touching or listening to voices more carefully. There are two types of resources for visually challenged persons i.e. non-optical aids and optical aids.

WHAT IS A BRAILLE SYSTEM: The most popular resource for visually challenged persons is known as Braille. It was developed by Louis Braille himself a visually impaired person. The present system was adopted in 1932. Braille system has 63 dot patterns or characters. Each character represents a letter, a combination of letters, a common word or a grammatical sign.

Light Class 8th Chapter 12 Questions

Exercises

1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.

Ans. No, we cannot see objects in a dark room because there is no light inside a dark room. We can see objects only in the presence of light. We can see things from which light comes or is reflected. If the light is present outside the room, we can see things. But if there is no light outside the room, we cannot see objects.

2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Regular Reflection	Diffused reflection
1. It is caused by reflection from smooth or shiny surfaces.	1. It is caused by reflection from rough or unpolished surfaces.
2. Reflected rays from regular reflection are parallel to each other.	2. Reflected rays from diffused reflection are not parallel to each other.

No, diffused reflection does not mean the failure of laws of reflection.

3. 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) a Piece of paper

Ans.

(a) Polished wooden table

A polished wooden table has a smooth surface so regular reflection takes place when light rays strike the surface of a polished wooden table.

(b) Chalk powder

Ans. The reflection from chalk powder will be diffused because chalk powder has very irregular surfaces from which reflected rays would not be parallel.

(c) Cardboard surface

Ans. The surface of the cardboard is uneven. So, it will show diffused reflection.

(d) Marble floor with water spread over it

Ans. The marble floor has a plain and smooth surface and water spread over it will give rise to regular reflection.

(e) Mirror

Ans. A mirror has a smooth surface so it will show regular reflection.

(f) a Piece of paper

Ans. A piece of paper has rough surfaces and thus shows diffused reflection.

4. State the laws of reflection.

Ans. There are two laws of reflection:

1. First Law of Reflection: This law states that incident ray, reflected ray and normal all lie in the same plane.
2. Second Law of Reflection: This law states that the angle of incidence is always equal to the angle of reflection. ∠i = ∠r.
3. 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.

Ans. The following activity describes that incident ray, reflected ray and normal all lie in the same plane.

First of all, fix a stiff paper on a table and let the sheet project a little beyond the edge of the table. Take a comb and close all its openings except one in the middle. Now hold the comb perpendicular to the sheet of paper. Project some light from a torch through the opening of the comb. You will observe a ray of light along the paper on the other side of the comb. Fix the comb and the torch steady, and put a strip of the plane mirror in the path of incident light. After striking the mirror, the ray of light is reflected in another direction. Now, cut the projected portion of the sheet in the middle and observe the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper. When a sheet of paper is spread on the table, then it shows that the incident ray reflected ray and normal all lie in the same plane.

6. Fill in the blanks in the following:

(a) A person 1 m in front of a plane mirror seems to be 2 m from his image.

(b) If you touch your left ear with your right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with your left hand.

(c) The size of the pupil becomes large when you see in dim light.

(d) Night birds have less cones than rods in their eyes.

Choose the correct option in Questions 7 – 8

7. Angle of incidence is equal to the angle of reflection

(a) Always

(b) Sometimes

(c) Under special conditions

(d) Never

Ans. (a) Always

8. Image formed by a plane mirror is

(a) virtual, behind the mirror and enlarged

(b) virtual, behind the mirror and of the same size as the object

(c) real at the surface of the mirror and enlarged

(d) real, behind the mirror and of the same size as the object.

Ans. (d) real, behind the mirror and of the same size as the object.

9. Describe the construction of a kaleidoscope.

Ans.

1. Take 3 rectangular strips of mirror each about 15 cm long and 4 cm wide.
2. Join the strips of mirror lengthwise to form a prism.Fix them with a few thick chart papers in a slightly longer circular cardboard or tube.
3. Close one end of the tube with a cardboard disc having a hole in the centre, through which one can see.
4. Paste a transparent plastic sheet under the cardboard to make it durable.
5. At the other end touching the mirrors fix a circular plane glass plate.
6. Invert the tube and place some small pieces of coloured glass on the glass plate.
7. Close this end of the tube with a round glass plate. And so, your kaleidoscope is ready to use.

10. Draw a labelled sketch of the human eye.

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 teachers’ advice?

Ans. Laser light comes from a very small hole and the lens of the laser light is also very small, so the light will penetrate one part of the eye and it can injure the retina. So, the beam of laser light should not be used to perform this activity.

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

Ans. We can take care of our eyes in the following manner:

1. Washing our eyes twice a day.
2. Do not keep the book too close to your eyes while reading.
3. Too little light or too much light is harmful to the eyes.
4. Consult an eye specialist in case of an injury.
5. Do not rub your eyes if something enters your eye. Wash it with water.
6. We should include those foods which contain vitamin A in our diet.

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

Ans. We know that according to the laws of reflection angle of incidence is equal to the angle of reflection.

Also ∠i + ∠r = 90⁰ (According to the question)

Since ∠i = ∠r

So, ∠i + ∠i = 90⁰

Or 2 ∠i = 90⁰

Or ∠i = 90/2 = 45⁰

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

Ans. An infinite number of images will be formed due to multiple reflections of light.

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.

Ans.

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

Ans. Yasir is not in the range of the reflecting surface of the mirror, so he cannot see himself in the mirror. But he can see the image of P which is in the reflecting area but not of Q and R.

17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).

(b) Can Paheli (Saba) at B see this image?

(c) Can Boojho (Yasir) at C see this image?

(d) When Paheli (Saba) moves from B to C, where does the image of A move?

Ans. Since it is a plane mirror, so the image is formed behind the mirror.

1. Yes, Paheli (Saba) at B see the image of object A.
2. Yes, Boojho (Yasir) at B see the image of object A.
3. There is no change in the position of image A

That’s all about Light Chapter 12 Class 8 Question Answers. Hope it has helped. Do share your views about this post in the comment section.