NCERT Solutions Class 8 Science Light

NCERT Solutions for Class 8 Science for chapter 16 Light

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

Answer: If we are in a dark room, we cannot see the objects kept inside the room. On the other hand, objects outside the room can be seen. An object becomes visible when light reaches our eye after being reflected from the object. If there is no light in the room, then the objects inside the room will not reflect any light. Hence, we cannot see in a dark room. If there is light present outside the room, then objects placed outside the room can be seen.

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

Answer: When all parallel rays reflected from a smooth, polished and regular surface are parallel, then the reflection is known as a regular reflection. Clear images are formed by regular reflection. Water and mirror are some of the examples of smooth surface. If all parallel rays reflected from an irregular surface are not parallel, then the reflection is known as an irregular or diffused reflection. It is caused by irregularities in the reflecting surface and we see hazy image or no image at all. A cardboard and a wall are some of the examples of irregular surface. In diffused reflection, each ray obeys the laws of reflection. Therefore, laws of reflections are not violated.

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) Piece of paper

Answer:

(a)    Polished wooden table (Regular reflection) Explanation: A polished wooden table acts as a smooth surface. Hence, reflections from the polished table will be regular.

(b)    Chalk powder (Diffused reflection) Explanation: Chalk powder is not an example of smooth surface. Therefore, diffused reflection will take place from chalk powder.

(c)    Cardboard surface (Diffused reflection) - Explanation: Cardboard surface is also an example of an irregular surface. Hence, diffused reflection will take place from a cardboard surface.

(d)    Marble floor with water spread over it (Regular reflection) - Explanation: Water makes marble surface smoother. Hence, regular reflection will take place from this surface.

(e)    Mirror (Regular reflection) - Explanation: Mirror has a smooth surface. Therefore, it will give a regular reflection.

(f)    Piece of paper (Diffused reflection) Explanation: A piece of paper has many irregularities on its surface. Hence, it will give a diffused reflection.


4. State the laws of reflection.

Answer: There are two laws of reflection:

(i) The angle of reflection is always equal to the angle of incidence.

(ii) The incident ray, the reflected ray and the normal to the reflective surface at the point of incidence all lie in the same plane.


5. 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:

image

Choose a place in the room where light is very dim. Place a plane mirror on the table. Take a paper sheet and make a small hole in its centre. Hold the sheet normal to the table. Take another sheet and place it on the table in contact with the vertical mirror. Now, draw a normal line on the second sheet from the mirror. Light a torch on the mirror through the small hole in such a way that the ray of light falls on the normal at the bottom of the mirror. When the ray from this hole is incident on the mirror, it gets reflected in a certain direction. Now, it can be observed that the incident ray, reflected ray and the normal to the mirror at the point of incidence 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         m away from his image.

(b) If you touch your           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         .

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

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

Answer:

(a)    A person 1 m far from a plane mirror seems to be 2 m away from his image.

Explanation: Object distance and image distance are the same from a plane mirror. Therefore, the image distance = 1 + 1 = 2 m away from the person.

(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.

Explanation: It is because lateral inversion.

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

Explanation: It is because the amount of light entering into the eye becomes very less. Hence, to increase the amount of light, the pupil expands.

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

Explanation: Night birds cannot see in the day but can see in the night. They have a few cones and a large number of rod cells on their retina.


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

(a)       Always

(b)       Sometimes

(c)      Under special conditions

(d)       Never

Answer: The correct option is (a).

Explanation: According to first law of reflection, the angle of incidence is always equal to the angle of reflection.


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.

Answer: The correct option is (b).

Explanation: The image formed by a plane mirror is of the same size as the object and is formed behind the mirror. The image cannot be obtained on a screen. Therefore, it is a virtual image.

9. Describe the construction of a kaleidoscope.

Answer:

Take three rectangular mirror strips each about 15 cm long and 4 cm wide. Group them with their long edges together to form a prism. Place and fix them inside a circular cardboard tube or a tube made up of a thick chart paper. Note that the tube must be slightly longer than the mirror strips. Cover one end of the cardboard tube by a cardboard disc. This disc must have a hole at its centre through which we can see. Paste a piece of transparent plastic sheet under the disc in order to make it durable. Fix a circular plane glass plate at the other end of the tube. Place some small pieces of coloured glass on this glass plate.    Cover this end of the tube with the ground glass plate and keep enough space for the coloured glass pieces to move. The kaleidoscope is ready. If we look through the hole of the kaleidoscope, beautiful image can be seen.


10. Draw a labelled sketch of the human eye.

Answer: The different parts of the human eye have shown here.

image


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

Answer: For human eyes, laser light is dangerous as its intensity is very high. It can harm to the retina. Therefore, it is advisabl6e not to look at a laser beam directly.


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

Answer: The given points should be taken into account to protect our eyes:

(i) Do not view the sun or the powerful lights directly.

(ii) When advised by doctor, use suitable and proper spectacles.

(iii) Too little and too much light is harmful for eyes. Little light leads to eyestrain and headaches while too much light like light from the sun, a powerful lamp or a laser torch can damage the retina.

(iv) We should never rub our eyes. When the particles of dust go into our eyes, then we should wash our eyes with clean water or else we should visit to a doctor.

(v) Maintain a distance of at least 25 cm between the book and your eyes while reading.


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: According to the law of reflection, the angle of incidence is equal to the angle of reflection.

Let, Angle of incident = i

Angle of reflection = r

As per law of reflection, i = r

∵          i + r = 90°

or,         i+i = 90°

or,        i = 90°/2 = 45°

Therefore, both, angle of incident and angle of reflection, will be 45°.


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 two mirrors are placed parallel to each other, then infinite numbers of images are formed due to multiple reflections between the mirrors.


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.

image

Answer: Here, the first law of reflection can be used to obtain the path of reflected light. The given ray of light will reflect from the second mirror at an angle 60°.

image


16. 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?

image

Answer:

A plane mirror always forms a virtual image behind the mirror and the image distance is equal to the object distance. Boojho cannot see his image because the length of the mirror is too short on his side. Though, he can see the objects placed at points P and Q, but cannot see the object placed at point R.

image

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 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:

image

(a) Image (A’) of the object (A) is formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror.

(b) Yes. Paheli can see this image at B. (c)    Yes. Boojho can see this image at C.

(d) Image (A’) of the object at A remain at the same position when Paheli moves from B to C.



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