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: 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: 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 (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: Laws
of reflection:
(i) The angle of reflection is always equal to the angle of
iincidence.
(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: 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.
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:
(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: (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: (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:
Construction of a kaleidoscope: Three rectangular mirror strips of
dimensions 15cm × 4cm (l × b) are joined together to form a prism (fig a). This
prism is fixed into a circular cardboard tube. The circular cardboard tube
should be slightly longer that the prism (fig b). This circular tube is now
closed at one end with a cardboard disc. This disc has a hole in it through
which we can see (fig c) .At the other end of the circular tube, a plane glass
plate is fixed. It is important that this glass plate touches the prism
mirrors. On this glass plate, several small and 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 (fig d).
Question 10: Draw a
labelled sketch of the human eye.
Answer:
Question 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?
Answer: Laser
light is harmful for the human eyes, because its intensity is very high. It can
cause damage to the retina and lead to blindness. Hence, it is advisable not to
look at a laser beam directly.
Question 12: Explain how
you can take care of your eyes.
Answer: To protect our eyes, the
given points should be taken into account:
(i) Visit an eye specialist regularly.
(ii) Avoid reading in dim light and very bright light.
(iii) Avoid direct exposure of sunlight to the eye.
(iv) Clean your eyes with cold water quickly if dust
particles or small insects enter your eye. Do not rub your eyes.
(v) Maintain a distance of at least 25 cm between the book
and your eyes while reading.
Question 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 an angle of 90° to the incident ray, then the angle of
incidence is 45 °. 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 = (90/2) = 45°.
Question 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.
Question 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°.
Question 16: Boojho stands
at A just on the side of a plane mirror as shown in Fig. 16.20. Can he see
himself in the mirror? Also can he see the image of objects situated at P, Q
and R?
Answer: A
plane mirror forms a virtual image behind the mirror. The image is as far
behind the mirror as the object is in front of it. A cannot see his image
because the length of the mirror is too short on his side. However, he can see
the objects placed at points P and Q, but cannot see the object placed at point
R (as shown in the given figure).
Question 17:
(a) Find out the position of the image of an object situated
at A in the plane mirror (Fig. 16.21).
(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 A from the mirror. Image
of A is shown in the given figure.
(b) Yes. Paheli at B can see this image.
(c) Yes. Boojho at C can see this image.
(d) Image of the object at A will not move. It will remain
at the same position when Paheli moves from B to C.
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