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
Polished wooden table – Regular reflection. A polished wooden table has smooth
surface. Hence reflections from the polished table will be regular.
Chalk powder – Diffused reflection. Chalk powder has irregular surface. Hence
diffused reflections will take place from chalk powder.
Cardboard surface - Diffused reflection. Cardboard surface has irregular
surface. Hence diffused reflections will take place from Cardboard surface.
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.
Mirror - Regular reflection. Mirror has smooth surface. Hence regular
reflection will take place.
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.
is ready. When we peep through the hole, we will be able to see a variety of
patterns in the tube.
Image from NCERT
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.
Image from NCERT