17 – Effects of Light
Q 1.
Fill in the blanks :
(1)
When the beams from the headlights of a car fall on an object in the night, the
shadows called…………... and............... can be seen.
(2)
During a lunar eclipse the shadow of the ..................... falls on the
............
(3)
During a solar eclipse the shadow of the………... falls on the ............
(4)
Various shades of colour are seen in the sky at sunrise and sunset due to
.............
Ans.(1) umbra, penumbra (2) earth, moon (3) moon, earth
(4) scattering of light
Q 2.
Explain the difference.
(1)
Point sources of light and extended
sources of light.
Ans.
Point sources of light Extended
sources of light
1. A
point source is tiny in size.
1. An extended source of light is bigger in
size.
2.The
umbra is obtained from this source of 2.
The umbra and penumbra are both
Light.
obtained from this source of light. Example : Light coming from a tiny
hole 3. Example : The sun, a torch
(2)
Umbra and penumbra.
Ans. Umbra
Penumbra
1. The
umbra is dark. 1. The
penumbra is faint.
2. The
umbra is obtained from a point 2.
The penumbra is obtained only from an
source
as well as an extended source of extended source of light.
light.
3. A
total eclipse takes place from the part 3. A partial eclipse takes place from the of of umbra.
part of penumbra.
Q. 6.
Answer the following questions in your own words:
(1) What is meant by scattering of light?
Ans.
The light rays hit the molecules, dust particles and other tiny particles
present in le atmosphere and get scattered. This phenomenon is called
scattering of light. When the scattered light rays enter our eyes, we perceive
the light. A beam of light, blue sky, reddish appearance of sun are all effects
of scattering of light.
(2)
Does the shadow really vanish in the zero shadow condition?
Ans.
On the zero shadow day, at noon, the sun reaches exactly over our head. Thus,
the shadow of our body in a standing position (without stretching hands and
legs) is formed right below the base of our feet. The shadow so formed is not
seen and hence appears to be vanished.
(3)
Will the laser beam be seen if it passes through a glass box which contains a
lighted incense stick?
Ans.
Scattering of light is required/necessary, for a ray of light to be visible.
For this, the light rays should get scattered on hitting the tiny particles in
its path. There are tiny particles of the incense stick scattered in the closed
glass box, due to which the laser rays get scattered. These scattered rays
enter our eyes and the laser beam is seen.
(4) Give
some examples of scattering of light that we come across in day-to-day life.
Ans.
(1) A beam of light emerging from a cinema projector on a screen. 2) A beam of
gnt emerging from head lamps of a car moving through a thick fog. (3) The sun
appears red at sunset. (4) The sky appears blue.
(5)
Why is the shadow of a bird flying high not seen on the earth?
Ans.
The sun is an extended source of light which is extremely bigger than the earth
When a bird flies (at relatively shorter height from the ground), there is a
formation of umbra and penumbra on the ground. But, as they fly high in the sky,
their umbra becomes smaller and smaller and eventually disappears. At the same
time, their penumbra becomes more and more fainter and then disappears. As a result, the shadow
of the bird flying high is not seen on
the earth.
(6)
Why is a penumbra not obtained from a point source?
Ans.
Scattering of light emerging from a point source does not take place. Thus,
only dark shadow of the object placed in front of the point source is obtained.
(7)
Various eclipses and the conditions during that period.
Ans.
(1) A solar eclipse : The moon comes between the sun and the earth in a
straight line. Thus, the shadow of the moon falls/casts on the earth. (See
figure on page 168.)
A
total solar eclipse - (i) It is visible from within the part of the earth where
the umbra of the moon falls. (ii) The solar disc is completely covered by the
moon. (iii) Darkness reads on the part of the earth where the shadow of the
moon falls.
A
partial solar eclipse - (i) It is visible from within the part of the earth
where the penumbra of the moon falls. (ii) The moon does not cover the solar
disc completely.
An
annular solar eclipse - (i) It is seen/visible from the part of the earth where
a very small portion of the umbra of the moon falls. (ii) It is seen that the
pat of solar disc,
except
the peripheral ring, is completely covered by the moon. (iii) The edge of solar
disc appears like an illuminated ring.
(2) A
lunar eclipse : The earth comes between the sun and the moon in a straight
line. Thus, the shadow of the earth falls/casts on the moon.
A
total lunar eclipse - At this event, the moon completely comes in the shadow of
the
une.
earth.
A
partial lunar eclipse - At this event, a part of the moon comes in the shadow
of the earth.
Q. 7.
Give scientific reasons :
(1)
Space beyond the earth's atmosphere appears dark.
Ans.
There is a vacuum beyond earth's atmosphere, due to which there is no medium
for scattering of sunlight. As a result, space appears dark.
(2) We
are able to read while sitting in the shade.
Ans.
The sun is an extended source of light which is far away from the earth. The
sunlight casts a shadow of the object, in its path, on the earth. This shadow
has no umbra but penumbra which is nothing but the shade. There is a sufficient
amount of light available to read in the shade. Hence, we are able to read while
sitting in the shade.
(3) We
should not observe the solar eclipse with naked eyes.
Ans.
The earth receives the harmful ultra-violet rays from the sun. During a solar
eclipse, even if brightness of the sun is less, the ultra-violet rays reach the
earth. If the solar eclipse is seen/watched with naked eyes, the ultra-violet
rays directly enter our eyes and damage vision. Hence, the solar eclipse should
not be observed with naked eyes.
(1)
Write a science based paragraph on What if the sun did not rise?
Ans.
If the earth stops revolving, the part of the earth opposite to the sun would
never face the sun. Thus, the sun would never rise at that part.
following
are its effects/consequences :
1.The part
of the earth opposite to the sun would not receive sunlight. That part would
always
experience darkness. Artificial sources of light would have to be used continuously.
For which, a large amount of electricity would need to be generated.
2.The
phenomena like solar eclipse, zero shadow, spectrum, various shades of colour
in the sky would not be seen.
3. Temperature
of that part of the earth would decrease extremely. Water present in the liquid
state on the earth would freeze completely. This would prove to be unfavorable
for the living world.
4.The
process of photosynthesis in the plants would not take place in the absence of sunlight.
This would adversely affect plant's growth. Animals that feed on plants would
become endangered. As a result, carnivorous animals that survive on these animals
would become extinct.
5.
Water from the oceans would not undergo the process of evaporation, due to
which
there
would be no rainfall. As a result, the amount of fresh water available on the
earth
would not increase. This would cause deficiency of water required for body.
6. Use
of solar energy would not be possible.
7.
Temperature of the part of earth facing the sun, would increase tremendously.
This
would
lead to faster evaporation of water and there would be no rainfall. As a
result. plant as well as animal life would come to an end.
Q (2) What efforts will you make to remove the
misconceptions about eclipses?
Ans. We will
make people understand and accept the fact that an eclipse is a natural
phenomenon like rain, spectrum, seasons. For which, we will take following
steps:
(1) We will provide the scientific information about
eclipses through various media.
(2) If an eclipse is going to take place in the near
future, we will create awareness about
it
through advertisements.
3. We
will give the experience of viewing an eclipse through special goggles and
telescopes.
4. We
will arrange a tour to observe an eclipse taking place in the region far away
from us.
5. We
will show people how an eclipse takes place by making models of the sun, the
moon and the earth.
6. We will
convince people that the intake of food during the period of eclipse does not
cause any ill effect on health.
