Light

Q69. 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.

(a) Polished wooden table – Regular reflection. A polished wooden table has smooth surface. Hence reflections from the polished table will be regular.

(b) Chalk powder – Diffused reflection. Chalk powder has irregular surface. Hence diffused reflections will take place from chalk powder.

(c) Cardboard surface - Diffused reflection. Cardboard surface has irregular surface. Hence diffused reflections will take place from Cardboard surface.

(d) Marble floor with water spread over it - Regular reflection. Marble floor with water spread over it is an example of regular surface. Hence regular reflection will take place.

(e) Mirror - Regular reflection. Mirror has smooth surface. Hence regular reflection will take place.

(f) Piece of paper - Diffused reflection. Although a piece of paper may look smooth, but it has many irregularities on its surface. Hence diffused reflections will take place from paper.

Q70. Describe the construction of a kaleidoscope.

Ans. To make a kaleidoscope, get three rectangular mirror strips about 15 cm long and 4 cm wide each. Join them together to form a prism. Fix them in a circular cardboard tube or tube of a thick chart paper. Make sure that the tube is slightly longer than the mirror strips. Close one end of the tube by a cardboard disc having a hole in the centre, through which we 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. Place on this glass plate several small pieces of coloured glass (broken pieces of coloured bangles). Close this end of the tube by a ground glass plate. Allow enough space for the colour pieces to move around. Kaleidoscope is ready. When we peep through the hole, we will be able to see a variety of patterns in the tube.

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Q71. 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. Fix a white sheet of paper on a drawing board or a table. Take a comb and close all its openings except one in the middle. Hold the comb perpendicular to the sheet of paper. Throw light from a torch through the opening of the comb from one side. With slight adjustment of the torch and the comb we will see a ray of light along the paper on the other side of the comb. Keep the comb and the torch steady. Place a strip of plane mirror in the path of the light ray. Let the sheet project a little beyond the edge of the Table. Cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper. Bend that part of the projected portion on which the reflected ray falls. Bring the paper back to the original position. When the whole sheet of paper is spread on the table, it represents one plane. The incident ray, the normal at the point of incidence and the reflected ray are all in this plane. When we bend the paper we create a plane different from the plane in which the incident ray and the normal lie. Then we do not see the reflected ray. It indicates that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

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