# Light (NCERT Solutions, Class 8)

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

Ans- The objects in the dark room can not be seen because we can see things when the light reflected from the objects and reaches to our eyes. But in the dark room there is no light. Therefore we can not see things inside the dark room.

The objects outside the room can be seen by us. Because, light of outside reflects from the objects and reaches to our eyes. Therefore, we can see objects outside the dark room.

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

Ans- Difference between the diffused and regular reflection are listed below:-

 Diffused reflection Regular reflection All parallel rays reflected from a plane surface are not parallel. All parallel rays reflected from a plane surface are parallel. Plane surface is not smooth like cardboard. Plane surface is smooth like mirror.

Irregular reflection is caused by the irregularities in the reflecting surface (not smooth) like cardboard surface. Therefore, diffused reflection does not mean the failure of the 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) Piece of paper

Ans-

 Surface on which beam of light strikes Diffused or regular reflection Reason Polished wooden table Regular reflection No irregularities in the surface or smooth surface Chalk powder Diffused reflection Irregularities in the surface Cardboard surface Diffused reflection Irregularities in the surface or smooth surface Marble floor with water on it Regular reflection No irregularities in the surface or smooth surface Mirror Regular reflection No irregularities in the surface or smooth surface Piece of the paper Diffused reflection Irregularities in the surface

(4) State the laws of reflection.

Ans-Laws of reflection

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

(ii) Incident ray, reflection ray and normal are at 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.

Ans-

Take a mirror and a sheet. Spread sheet and keep mirror at the edge of the sheet. Keep light dim in room. Now, light a torch through a hole in the piece of cardboard or thick sheet. An incident ray reflected from the mirror. Draw these lines. Their meeting point is point of incidence. A perpendicular from this point is known as normal. Hence, the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

(6) Fill in the blanks

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

(b) If you touch your ____ ear with 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.

Ans-

(a) A person 1 m in front of a plane mirror seems to be 2 m away from his image. (In plane mirror, image obtained at the exact distance from the object. Hence, total distance from the object is 1 + 1 = 2 m)

(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 right.

(c) The size of the pupil becomes large when you see in dim light. (In dim light our pupil expands to adjust with light to see the objects)

(d) Night birds have a few 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- (b) virtual, behind the mirror and of the same size as the object.

(9) Describe the construction of a kaleidoscope.

Ans – To make kaleidoscope take three rectangular mirror strips of same size. Join them to make a prism. Fix a glass plate at one end. Now put several pieces of glass, shining dust etc. Now fix a piece of cardboard with a hole to the other end. Now you can see many patterns inside the kaleidoscope.

(10) Draw a labeled sketch of the human eye.

Ans-The labeled sketch of the human eye is drawing below:-

(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?

Ans- Teacher’s advice is absolutely correct because a laser torch can harm our retina. Therefore, Gurmit should not perform Activity 16.8 with a laser torch.

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

Ans- Care of the eyes

(i) If you have difficulties in reading or seeing objects, use suitable spectacles.

(ii) Proper light is very necessary for eyes. Less light causes headache and eyestrain. The sharp light or power lights like sunlight, a laser torch etc. are also very harmful for eyes. These power lights can injure retina of eyes.

(iii) Do not look at the sun or a powerful light directly.

(iv) If any foreign particles go in to the eye, wash your eyes with clean water immediately. But never rub your eyes and go to the doctor.

(v) Always wash your eyes frequently with clean water.

(vii) Always have vitamin A rich diet like raw carrot, broccoli and green vegetables. Lack of vitamin A causes night blindness.

(13) What is the angle of incidence of a ray if the reflected ray is at an angle of 900 to the incident ray?

Ans- According to law of the reflection

Angle of incident ray = Angle of the reflected ray

Angle between incidence ray and reflected ray = 900

2  angle of incidence = 900

Angle of incidence = 900/2

= 450

Hence, angle of the incidence is 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-

Infinite images will be formed because of the multiple reflections if candle is placed between two parallel plane mirrors separated by 40 cm.

(15) Two mirrors meet at right angles. A ray of light is incident on one at an angle of 300 as shown in figure below. Draw the reflected ray from the second mirror.

Ans- The reflected ray from the second mirror is showing below –

(16) Boojho stands at a just on the side of a plane mirror as shown in figure below. Can he see the image of objects situated at P, Q and R?

Ans-

Boojho can not see himself and images of objects at R in the mirror as they are not in front of the mirror. Therefore, incidence ray will not reflect from the mirror to Boojho’s eyes. But, Boojho can see objects at P and Q because P and Q are in front of the mirror. Therefore, incidence rays of object at P and at Q will reflect from the mirror to Boojho.

(17) (a) Find out the position of the image of an object situated at A in the plane mirror (Figure below)

(b) Can Paheli at B see this image?

(c) Can Paheli at C see this image?

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

Ans-

(a) The position of the image of an object situated at A in the plane mirror is showing below-

(b) Yes, Paheli at B can see this image because she is in front of the mirror and incidence ray will reflect from the mirror and reaches to the Paheli as draw in fogure above.

(c) Yes, Boojho can also see this image as he is also in front of the mirror and incidence ray will reflect from the mirror and reaches to the Boojho as draw in fogure above.

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

Helping Topics

Light

Practice sheet, class 8