At one moment, the stars
appear to be bright and the next moment, it become
very dim and we said that the stars are twinkling at
night. This phenomena of twinkling of a star is caused
by the atmospheric refraction.
Explanation- When the light coming from a star enters
the earth's atmosphere then it undergoes refraction
due to to various optical densities of air at various
altitude. the continuously changing atmosphere
refract the light from the star by different amount
from one moment to the next.
When the atmosphere refract more star light towards
us then the star appears to be bright and when the
atmosphere refractive less star light then the star
appears to be dim. the starlight reaching our eyes
increases and decreases continuously due to
atmospheric refraction and this phenomenon is called
twinkling of a stars.
Note:- Distant star act like a point source of light.
Q) Why planets do not twinkle?
-> Planets are closer to earth and are seen as
extended source of light i.e. the collection of large
number of point sized sources of light. Therefore the
total amount of light entering our eyes from all
individual point source will nullify the twinkling effect.
* Advance sunrise and Delayed sunset- It is also
caused by atmospheric refraction. Because of this, the
sun is visible about 2 minute earlier than actual
sunrise and 2 minute after than actual sunset.
The actual sunrise takes place when the sun is just
above the horizon.
But due to refraction of sunlight
caused by atmosphere, we can see sun rising before 2
minute when it is actually below the horizon.
When the sun is slightly below the horizon, the sun's
light coming from less dense air to more dense air is
refracted towards as it passes through the
atmosphere and due to atmospheric refraction, the
sun appears to be raised above the horizontal then it
is actually below the horizon.
Note:- So, the time from sunrise to sunset is
lengthened by about 4 minutes because of
atmospheric refraction.
-> The sun appears flattened or oval shape at sunrise
and sunset. This apparent flattening of the sun at
sunrise and sunset is also caused by atmospheric
refraction.
Scattering of light
-> It means to diverse light in various random
directions. Light is scattered when it falls on various
types of suspended particles in its path.
According to Rayleigh’ Law of Scattering,
-> The scattering of light by a medium containing a
small suspended particles is called Tyndall effect.
e.g. of such medium are:- dust particles in a room,
smoke in the atmosphere, etc.
Explanation- When a beam of light enters in a dusty
room through a hole or a window then its path
become visible to us. This is because the tiny dust
particles present in the air of room scattered the
beam of light all around the room and when the
scattered light enters in our eyes, then we can see the
beam of light clearly.
Note:- The colours of the scattered light depend on
the size of the scattering particles.
Note:- Blue light has shorter wavelength, So it is
scattered more easily. Red light has longer
wavelength, So it is not scattered much. The blue
coloured light present in white sunlight is scattered
much more easily than the red light.
* Dust particles and water droplets suspended in the
atmosphere are much larger than the wavelength range
of visible light So, When white light coming from the
sun hits these larger particles, it gets reflected or
scattered in different directions.
The different colours of white light are reflected by
the dust and water particles in the same way.
Due to this, the scattered light appears white. Thus,
when white sunlight falls on larger particles (like dust
particles and water droplets) present in the
atmosphere, then the cloud & sky appears white.
* The air molecules (nitrogen and oxygen gas molecules)
present in the atmosphere are smaller than the
wavelength range of visible light.
Since the different
colours of white light have different wavelength, So
When light coming from the sun hits these very small
air molecules, they behaves differently.
The lower wavelength lights (blues) are scattered
much more by the air molecules but the
higher wavelength lights (reds) are scattered much
less.
When white sunlight falls on the
extremely small particles like air molecules present in
the atmosphere, it is not scattered as
white light. The molecules of air scatter mainly the
lower wavelengths of light which have blue shades.
That’s why the sky appear blue in colour.
Explanation- The sky appears blue because the
molecules in the air (nitrogen and oxygen molecules)
scatter blue part of the sunlight much more than they
scatter red light (or other shades).
Note:- Only a little of the blue light present in white
sunlight is scattered by the atmosphere which
makes the sky appear blue. Most of the blue light
remains behind unscattered due to which
the composition of sunlight remains almost unaltered.
Because of this, the direct sunlight coming through
the blue sky still appears to be white.
* If the earth had no atmosphere consisting of air,
there would have been no scattering of sunlight at all.
In that case, no light from the sky would have entered
in our eyes and the sky would have looked dark and
black to us.
e.g. During Night
* In outer space, the sky looks dark and black instead
of blue. This is because there is no atmosphere
containing air in the outer space to scatter sunlight.
Since there is no scattered light to reach our eyes in
outer space, therefore, the outer space looks dark and
black to us.
Colour of Sunrise and Sunset
-> While sunset and sunrise, the colour of the sun and
its surroundings appear red. During sunset and
sunrise, the sun is near to horizon, and therefore, the
sunlight has to travel larger distance in atmosphere.
Due to this, most of the blue light that has shorter
wavelength is scattered away by the particles.
The
light of red colour that has longer wavelength reaches
in our eyes. As a result, sun appear red in colour
during sunrise and sunset.
-> Sun appears white when it is overhead in the sky or
at noon, When the sun is overhead or at noon ,then
the light coming from the sun has to travel a relatively
shorter distance through the atmosphere to reach us.
During this shorter journey of sunlight, only a little of
the blue colour of the white light is scattered and
most of the blue light remains in white light. Since the
light coming from the overhead sun has almost all its
component colours in the right proportion, therefore,
the sky appears white at noon.
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