NCERT Solution (Part - 1) - Light Class 8 Notes | EduRev

Science Class 8

Class 8 : NCERT Solution (Part - 1) - Light Class 8 Notes | EduRev

The document NCERT Solution (Part - 1) - Light Class 8 Notes | EduRev is a part of the Class 8 Course Science Class 8.
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Question 1: Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.
Answer 1: If we are in a dark room, then it is not possible for us to see objects in the room. However, objects outside the room are visible to us.
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 cannot reflect any light. Hence, we cannot see in a dark room. If there is light present outside the room, then we can see the objects outside the room.

Question 2: Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Answer 2: Regular reflection takes place from a smooth or a regular surface. In regular reflection, all reflected rays are parallel to each other for parallel incident rays. Irregular or diffused reflection takes place from an irregular surface. In diffused reflection, the reflected rays are not parallel to each other for parallel incident rays. This happens because of the presence of irregular microscopic surfaces. Hence, parallel incident rays reflect in different directions. However, each ray obeys the laws of reflection. Therefore, laws of reflections are not violated in diffused or irregular reflections.

Question 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 3:
(a) Polished wooden table: Regular reflection

A polished surface is an example of a smooth surface. A polished wooden table has a smooth surface. Hence, reflections from the polished table will be regular.
(b) Chalk powder: Diffused reflection
Chalk power spread on a surface is an example of an irregular surface. It 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 a regular surface. This is because water makes the marble surface smooth. Hence, regular reflection will take place from this surface.
(e) Mirror: Regular reflection
Mirror has a smooth surface. Therefore, it will give a regular reflection.
(f) Piece of paper: Diffused reflection
Although a piece of paper may look smooth, but it has many irregularities on its surface. Due to this reason, it will give a diffused reflection.

Question 4: State the laws of reflection.
Answer 4: 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.

Question 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 5: Place a plane mirror on the table. Take a paper sheet and make a small hole in its centre. Make sure that the light in the room is not bright. Hold the sheet normal to the table. Take another sheet and place it on the table in contact with the vertical mirror. Draw a normal line on the second sheet from the mirror. Now, light a torch on the mirror through the small hole such 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. You can easily observe the incident ray, reflected ray and the normal to the mirror at the point of incidence on the sheet 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.
NCERT Solution (Part - 1) - Light Class 8 Notes | EduRev

Question 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 6:
(a) A person 1 m in front of a plane mirror seems to be 2 m away from his image.

(Object distance and image distance are the same from a plane mirror. The image of a person 1 m in front of a mirror is 1 m back to the mirror. Hence, the image is 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
(This is because of lateral inversion of images formed in a plane mirror.)
(c) The size of the pupil becomes large when you see in dim light.
(In dim light, the amount of light entering the eye is very little. To increase the amount of light, the pupil expands.)
(d) Night birds have less cones than rods in their eyes.
(Night birds can see in the night, but not in the day. They have on their retina a large number of rod cells and only a few cones.)

Question 7: Angle of incidence is equal to the angle of reflection.
(a) Always 
(b) Sometimes

(c) Under special conditions
(d) Never

Answer 7: (a) The angle of incidence is always equal to the angle of reflection. This is the first law of reflection.

Question 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 8:
(b) Image formed by a plane mirror is virtual, behind the mirror and of the same size as the object. The image formed by a plane mirror is of the same size as the object. The image is formed behind the mirror. The image cannot be obtained on a screen. Hence, it is a virtual image.

Question 9: Describe the construction of a kaleidoscope.
Answer 9: Construction of a kaleidoscope:
To construct a kaleidoscope, take three rectangular mirror strips about 15 cm long and 4 cm wide each. Join them together to form a prism. These are fixed in a circular cardboard like tube or tube of a thick chart paper. The tube must be slightly longer than the mirror strips. One end of the tube remains closed by a cardboard disc having a hole in the centre, through which one can see. To make the disc durable, paste a piece of transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, fix a circular plane glass plate. On this glass plate, several small broken pieces of coloured glass are placed. This end is now closed by a round glass plate allowing enough space for the coloured glass pieces to move.

Question 10: Draw a labelled sketch of the human eye.
Answer 10:
NCERT Solution (Part - 1) - Light Class 8 Notes | EduRev

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