21. Why do stars twinkle but not the planets?
The density of the atmosphere is not uniform as it goes on
decreasing with the distance from the earth. The rays coming
from the distant stars have to pass through the various
mediums in which the light gets refracted due to the variation
of densities. Also the atmosphere in the earth is not
stationary due to which the position of the star changes much
faster from the earth. Hence, the change in the position of
the star creates the twinkling of the star.
But the distance betweens the planet and the earth is much
more less than the distance between the earth and the star.
So, there is negligible change in the
position of the planet from the earth due to which it does not
appear to be twinkling at the sky.
22. Why does a crack in glass plane appear shining when viewed
from a suitable direction?
In a crack glass, the air replaces the cracked volume. When
it is observed from the suitable direction, the light of it
has to pass from two medium from the glass to the air i.e.
from the denser to the rarer medium. during their journey they
make incidence angle greater than the critical angle due to
which the total internal reflection process occurs. Hence
these rays appeared to be shining when viewed from a suitable
directions. So, crack appears to shine.
23. If you see a fish in clear water, how should you aim to
shoot it? Explain.
As the light rays coming from a fish passes through
two medium i.e. from denser to rarer medium. As the light
passes from denser to rarer medium, according to the law of
refraction, it goes away from the normal. Due to this the fish
appear to be nearer to the surface i.e. it appears in its
apparent position. In order to shoot it we should aim at a
position below than the apparent position which is the actual
position of the fish.
24. Why does powdered glass appear white and opaque but becomes
transparent when water is poured over it? Explain.
As the surface of a powdered glass is not regular but is
rough and irregular so, irregular or diffused reflection
occurs due to which the powdered glass appears white and
opaque.
But when the water is poured over it, almost all rays are
refracted instead of reflection due to the refractive index of
water (µ = 1.33)
and glass (µ = 1.5)
being nearly equal. So, due to the formation of the media the
light undergoes the refraction phenomena instead of the
reflection due to which the powdered glass appears transparent
when water is poured in it.
25. What does a fish see when it looks towards the sky?
When the fish looks towards the sky it observes the sky
portion in circular
hole where the range of the angle is critical angle and beyond
that circle it observes the objects of water due to the total
internal reflection. If the critical angle is c and the fish
is at height h from the water surface then
Now,
where, r is
the radius of the circular hole observed by the fish
µ
is the
refractive index of water.
Or, r = h(tanc)
26. A fish under water is viewing a tree obliquely on the bank
of river. Does the tree look shorter or taller than its actual
height?
When a tree in the air medium i.e. rarer medium is viewed by
a fish under water i.e. from the denser medium, the apparent
height of the tree is observed by a fish which is greater than
the real height of the tree. It is due to the phenomena of
refraction when the light traveled from the denser to rarer
medium, it goes away from the normal. So, the tree looks
taller than its actual height.
27. Why does a
diamond sparkle with a great brilliancy?
The refractive index of the diamond is very high and its
critical angle is about
. The faces of the diamond are cut in such a way that when a
light enters into it, the angle of incident is always greater
than the critical angle, which results in the multiple total
internal reflection. So, due to this the diamond sparkle with
great brilliancy.
28.
Why does a clear pool of water appear to be shallower than it
actually is?
Due to the refraction of light, the light rays coming from
the bottom of the pool bends away from the normal. Hence the
light rays actually coming from the depth of the pool appears
as coming from the lower depth than actual depth. So, the
depth of the pond appears slightly raised due to which a clear
pool of water appear to be shallower than it actually is.
29. If a plane glass slab is placed on letter of different
colors, then violet colored letters appear more and more raised
up. Why?
The vertical distance by which a body or anything rises
as the introduction of a refracting medium is given by
The refractive index in turn is given by
30. Why do the trees and houses on the other side of an open
ground appear to be shaking during the summer noon?
During the summer noon, the air in contact with ground gets
heated. When temperature rise, the density of air is decrease
as the density varies inversely with the temperature. as a
result of which there is a movement of hot air towards upward
and the cold air downward creating the difference in the
densities. So, when light rays pass via it, the path of light
rays are changed due to refraction. Due to the fluctuations of
air above the ground produce shaking appearance of the tree
and houses on the side of an open ground.
31. How does the refraction affect the length of the day?
If there is no atmosphere on the earth surface, the length
of the day decreases by 4 minutes i.e. 2 minutes in morning
& 2 minutes in evening. Hence, Due to refraction of light
through the atmosphere, the length of the day increases by 4
minutes.
32. A coin placed at the bottom of a beaker containing water
seems to be raised. Explain why?
When light rays of a coin from the bottom of water is
observed by the observer in the air then the presence of the
two media results in the refraction of the light. When light
rays travel from the water (denser medium ) to air (rarer
medium), bends away from the normal. So, the refracted rays
coming from a coin is observed at its apparent position rather
than its real position. Hence, a coin placed at the bottom of
a beaker containing water seems to be raised.
33. Why does the sun look a little oval when it is at the
horizon?
The
refraction of the light due to the variation in the density of
the air in the atmosphere is the main cause for the oval shape
of the sun when it is at the horizon. The magnitude of
refraction increases with decreasing the altitude. As a result
of which the lower portion of the sun are raised much than the
upper position. So, the vertical diameter is shortened more
than its horizontal diameter. Hence the sun acquires the oval
shape at the horizon.
34. Swimming pools are always deeper than they look. Why?
When the swimming pool is observed, the light rays of the
foot of the pool have to pass through the two media from water
to air i.e. from denser to rarer medium. As the passes from
denser to rarer medium, it goes away from the normal. Hence
the original or real depth of the pool is observed at its
apparent position where it is raised. So, swimming pools are
always deeper than they look.
35. Why does an air bubble in a jar of water shine brightly?
Light rays hitting the edges of the air bubble make the
incident angle always greater than the critical angle which
results in the total internal reflection. Due to it, an air
bubble in a jar shine brightly.
36.
The sun is visible a even before the actual sunrise and after
actual sunset. Explain.
The density of air decreases as the distant is increased
from the earth in the atmosphere. The magnitude of refraction
increases with decrease in the height. Due to this, the lower
portion of the sun at the time of sun set and sunrise are
raised more than the upper portion. So, the vertical diameter
of the sun is shortened more than the horizontal diameter. As
a result, the sun is visible a even before the actual sunrise
and after actual sunset.
37. Why one prefers a black umbrella even in summer?
A black umbrella absorbs all the radiations of the sunlight
and it provide better shade, while a white umbrella is
not preferred because a part of the sunlight may penetrate
through it which may be harmful, So, one prefers a black
umbrella even in summer.
38. A stick partially dipped in water seems to be bent. Why?
When light rays passes from denser medium to rarer medium,
then rays bent away from the normal. Due to the refraction of
the light, the rays coming from the bottom of the stick dipped
goes away from the normal which results in the bending of a
stick when dipped in water.
39. Why do in summer roads often appear to be covered with water
when seen from a distance? Explain.
In the
pitch road due to heat of the sun, the layer near the road
becomes hot and it behaves as a rarer medium. And due to the
total internal reflection, a mirage ( which is an optical
illusion ) is seen in the pitch road during the summer. A ray
of light coming from the road passes successively from denser
to rarer layers and bends away from the normal. When the angle
of incident is greater than the critical angle, the ray gets
totally internally reflected and reaches the eyes of the
observer who sees the roads covered with water. Hence,
in summer roads are often appears to be covered with water
when seen from a distance.
40. Which one of blue and red lights will be deviated more by a
prism and why?
The deviation produced by a small angled prism is given by
the relation:
where, A is the
angle of the prism and
is the refractive index of the material of the prism. As the
refractive index increases with decrease in the wavelength,
the blue light has more refractive index than that of red
light. The larger the refractive index, the greater is the
deviation. Hence, blue light will be deviated more than red
light by a prism.
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