NCERT Solutions for Class 8 Science Chapter 16 Light

Q.1 Suppose you are in a dark room. Can you see objects in the room? Can you see objects out¬side the room ?

Answer :

If we are in a dark room, then it is not possible to see objects in the room. However, objects outside the room are visible to us.

An object becomes visible when the reflected light from the object reaches our eye. If there is no light in the room, then the objects inside the room cannot reflect any light.

Hence, we cannot see anything in a dark room. But if there is light outside the room, then we can see the objects outside the room.

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

Answer :

Regular Reflection

(i) All the reflected rays are parallel to each other.
(ii) Regular reflection takes place when the reflection occurs on a smooth and polished surface.
(iii) Reflected rays travel in one direction.

Diffused reflection

(i) The reflected rays are not parallel to each other.
(ii) Diffused reflection takes place when the reflection occurs on a rough surface.
(iii) Reflected rays travel in scattered directions.

Diffused reflection doesn't means the failure of the laws of reflection. As it occurs only when the reflecting surface is rough and has irregularities, so it doesn't violate the laws of reflection.

Q.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

A polished surface is an example of a smooth surface, so a polished wooden table has a smooth surface. Hence, reflections from a smooth surface will be regular.

(b) Chalk powder - Diffused reflection

Chalk power spread on a surface is an example of an irregular surface. As the surface is not smooth, therefore diffused reflection will take place from chalk powder.

(c) Cardboard surface - Diffused reflection

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

Marble floor with water spread over it is an example of smooth surface. This is because water makes the marble surface more smooth. Hence, regular reflection will take place from this surface.

(e) Mirror - Regular reflection

Mirror has a smooth surface. Hence, regular reflection will take place from this surface.

(f) Piece of paper - Diffused reflection

Though a piece of paper looks smooth, but it has many minute irregularities on its surface. Due to this diffused reflection will take place from this surface.

Q.4 State the laws of reflection.

Answer :

The laws of reflections are as follows:

(1) The incident ray, the normal and the reflected ray, all lie in the same plane.
(2) The angle of incidence is equal to the angle of reflection.

Q.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 :

Place a plane mirror on the table in a standing position. Take a piece of paper and make a small hole in its centre. Make sure that the light in the room is not so bright. Hold the sheet normal/ perpendicular to the table. Take another large piece of paper and place it on the table horizontally in contact with the vertical mirror. Draw a normal line on the large piece of paper from the mirror.

Now, light a torch on the mirror through the small hole such that the ray of light falls on the normal/ perpendicular line that touches the bottom of the mirror. When the ray from this hole is incident on the mirror, it gets reflected in a certain direction.

Now we can easily observe the incident ray, reflected ray and the normal to the mirror at the point of incidence on the large piece of paper placed on the table. This shows that the incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.

Q.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 a right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with _____
(c) The size of the pupil becomes _______ when you see in dim light.
(d) Night birds have _______ cones than rods in their eyes.

Answer :

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

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

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

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

EXPLANATION

A person 1 m in front of a plane mirror seems to be 2 m away from his image. Because his image will be 1 m from the mirror and he is 1 m from the mirror. So distance between image and object(person) is 1m + 1m = 2 m

If we touch our left ear with right hand in front of a plane mirror it will be seen in the mirror that our right ear is touched with the left hand. This is because of lateral inversion of images that takes place in a plane mirror.

The size of the pupil becomes large when we see in dim light because our pupil expands to allow more light to enter our eye.

Cones function best in relatively bright light, while rod cells work better in dim light.

Q.7 The angle of incidence is equal to the angle of reflection
(a) Always
(b) Sometimes
(c) Under special conditions
(d) Never

Answer :

The angle of incidence is equal to the angle of reflection
(a) Always

EXPLANATION

This is the law of reflection which states that the angle of incidence is always equal to the angle of reflection.

Q.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 :

Image formed by a plane mirror is
(b) virtual, behind the mirror and of the same size as the object.

EXPLANATION

A real image is the collection of focus points made by converging rays, while a virtual image is the collection of focus points made by extensions of diverging rays. In case of plane mirror, image is formed behind the mirror and the size of both the object and the image is the same.

Q.9 Describe the construction of a kaleidoscope.

Answer :

Construction of a kaleidoscope:

Three rectangular mirror strips of dimensions 15cm x 4cm (l x b) are joined together to form a prism (Fig a).

This prism is fixed inside a circular cardboard tube where the length of the circular cardboard tube should be slightly longer than the prism (Fig b).

This circular tube is now closed at one end with a cardboard disc and the disc has a hole in it through which we can see the inside of the tube(Fig c).

At the other end of the circular tube, a plane glass plate is fixed in such a way that this glass plate touches the prism mirrors. Several small and broken pieces of coloured glass are put in this glass plate. This end is then closed by a round glass plate allowing enough space for the coloured glass pieces to move (Fig d).

Q.10 Draw a labelled sketch of the human eye.

Answer :

Q.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 advise?

Answer :

Her teacher advised her not to do so because of the intensity of the laser light is very high which is harmful to the human eyes.

It can cause damage to the retina and which can lead to permanent blindness.

Hence, it is also advisable not to look at a laser beam directly.

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

Answer :

Different measures taken to protect our eyes are as follows:-

(i) We should visit an eye specialist on a regular basis.

(ii) We should avoid reading or writing in very dim light and very bright light.

(iii) We should keep a distance of atleast 25 cm between the eyes and the book while reading.

(iv) We should avoid direct exposure of eyes to the sunlight.

(v) We should quickly clean our eyes with cold water if dust particles or small insects enters our eye and avoid rubbing it with our hands.

Q.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 :

If the reflected ray is at the angle of 90 degrees to the incident ray, then the angle of incidence is 45 degrees.

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

Therefore, the angle of incidence and the angle of reflection both are equal to
90/2 = 45 degrees.

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

Answer :

Infinite or multiple images of the candle will be formed because of multiple reflections between the mirrors.

When two mirrors are placed parallel to each other, then infinite numbers of images are formed.

Example of infinity candle

Q.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.

Answer :

The first law of reflection is used to obtain the path of reflected light.

It can be observed that the given ray of light will reflect from the second mirror at an angle 60°.

Q.16 Boojho 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?

Answer :

No, Boojho can’t see himself in the mirror. As the reflected ray of his image formed behind the mirror won't reach his eyes, so he won't be able to see himself in the mirror.

He can see the image of the object at P and Q because the reflected ray of their image can reach his eyes while he cannot see the image of the object at R as the reflected ray of the image at R cannot reach his eyes.

Q.17 (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.23).
(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) Image of the object placed at A is formed behind the mirror.
The distance of the image from the mirror is equal to the distance of object from the mirror. Image of A is shown in the given figure.



(b) Yes, Paheli can see the image of A.

(c) Yes, Boojho can see the image of A.

(d) When Paheli moves from B to C, the image of the object at A will not move as the object is not moving.