Science – Jammu and Kashmir | jandkncert | Class 8th |
Light | Science 8th | Chapter 12 |
Chapter 12: Light
Q1. Suppose you are in a dark
room. Can you see objects in the room? Can you see objects outside the room? Explain.
Ans. 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 it in a dark room. If there is light present outside the room, then we can see the objects outside the room.
Q2. Differentiate between
regular and diffused reflection. Does diffused reflection mean the failure of
the laws of reflection?
Ans. Regular reflection takes place
from a smooth or 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.
Q3. 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 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 powder spread on a surface is
an example of an irregular surface. It is not smooth. Therefore, the diffused
reflection will take place from chalk powder.
(c) Cardboard surface → Diffused
reflection
The cardboard surface is also an
example of an irregular surface. Hence, the diffused reflection will take place
from a cardboard surface.
(d) Marble floor with water spreads
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, the 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, it has many irregularities on its surface. Due to this reason, it
will give a diffused reflection.
Q4. State the laws of
reflection.
Ans. 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.
Q5. 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. Place a plane mirror on the
table. Take a paper sheet and make a small hole in its center. 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.
 |
| Figure showing point of incidence of the incident ray, normal and reflected ray |
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Q6. 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.
Ans. (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 the 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 fewer
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.)
Q7. The angle of incidence is
equal to the angle of reflection.
(a) Always (b) Sometimes
(c) Under special conditions
(d) Never
Ans. (a) The angle of incidence is
always equal to the angle of reflection. This is the first law of reflection.
Q8. The 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) The 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.
Q9. Describe the construction
of a kaleidoscope.
Ans. 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 than 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).
 |
| Fig. Kaleidoscope |
Q10. Draw a labeled sketch of the human eye.
Ans.
Q11. 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. Laser light is harmful to 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.
Q12. Explain how you can take
care of your eyes.
Ans. 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.
Q13. What is the angle of
incidence of a ray if the reflected ray is at an angle of 90° to the incident
ray?
Ans. 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 900/2= 45°.
Q14. How many images of a
candle will be formed if it is placed between two parallel plane mirrors
separated by 40 cm?
Ans. 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.
Q15. 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.
Ans. 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 of 60°.
Q16. Yasir 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?
Ans. 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).
Q17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).
(b) Can Saba at B see this
image?
(c) Can Yasir at C see this
image?
(d) When
Paheli moves from B to C, where does the image of A move?
(c) Yes. Yasir at C can see this image.
(d) The image of the object at A will not move. It will remain in the same position when Paheli moves from B to C.
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