Rainbow
Definition-It is the spectrum of sunlight in nature. It is formed due to the dispersion of sunlight by the tiny water
droplet, present in the atmosphere.
Formation of the Rainbow:
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The water droplets act like small prism.
They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again
when it comes out of the raindrop.
Due to the dispersion of light and internal reflection, different colours reach the observer’s eye.
Red colour appears on top and violet at the bottom of rainbow.
A rainbow is always formed in a direction opposite to that of Sun.
Phenomena Involved- Refraction
Dispersion
Total Internal Reflection
Refraction
Condition for formation of Rainbow. Sun must be at the back of the observer and rain should be
falling in front of the observer.
At ‘A’ – Refraction and dispersion take place.
At ‘B’ – Internal reflection takes place.
At ‘C’ – Refraction and dispersion take place.
Atmospheric Refraction
The refraction of light caused by the Earth’s atmosphere (having air layers of varying optical densities) is
called Atmospheric Refraction.
Phenomena involved on the Atmospheric Refraction
1-Appearance of Star Position (star appears slightly higher):
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It occurs due to atmospheric refraction of light coming from the star.
A distant star behaves like a point source of light.
The Earth's atmosphere consists of layers with continuously changing density, caused by
variations in temperature and pressure.
These layers, having different densities, exhibit different refractive indices.
As starlight enters the Earth's atmosphere, it undergoes continuous refraction due to the changing
refractive index—from rarer to denser layers—causing the light to bend towards the normal.
As a result, the apparent position of the star differs from its actual position and the star appears
slightly higher than where it truly is.
2-Twinkling of Star:
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Stars appear to twinkle due to atmospheric refraction.
A distant star acts like a point source of light.
The temperature and pressure in the Earth's atmosphere keep changing, which causes continuous
variation in the density and refractive index of its layers.
As a result, the path of light coming from the star bends continuously while passing through these
layers.
This bending causes the apparent position of the star to shift slightly and constantly.
Because the star’s apparent position keeps changing, the amount of light entering our eyes also
varies—sometimes more, sometimes less—making the star appear to twinkle.
3- 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 change in total amount of light entering our eyes from all individual points source of
planets will average out to be zero and hence nullify the twinkling effect.
4-The duration of day becomes approximately 4 minutes longer due to Atmospheric
Refraction.
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Sunrise before the Sun appears: We see the Sun a little earlier than its actual rise because the
atmosphere bends (refracts) its light, making it visible even when it is still below the horizon.
Sunset after the Sun disappears: Even after the Sun has gone below the horizon, its light bends through
the atmosphere, so we continue to see it for a short while.
Daytime becomes longer: This bending of light makes the Sun appear about 2 minutes earlier at sunrise
and 2 minutes later at sunset, adding nearly 4 extra minutes to the day.
Flattened Sun near the horizon: The Sun looks slightly squashed or flattened during sunrise and sunset
due to this same bending of light (atmospheric refraction).
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Scattering of light
Definition-Scattering of light is a phenomenon in which a beam of white light traveling through a
medium gets deviated from its straight path and spreads in different directions when it interacts
with small particles, molecules, or irregularities present of the medium.
The extent and nature of scattering depend on the size of the particles and the wavelength
of the light.
The colour of the scattered light depends on the size of the scattering particles.
Role of Particle Size in Scattering of light:
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Very small particles (smaller than wavelength): Cause Rayleigh scattering – strong for
shorter wavelengths (blue/violet). Example: Blue sky.
Particles similar in size to wavelength: Cause Mie scattering – affects all wavelengths
more evenly. Example: White appearance of clouds.
Larger particles: Cause non-selective scattering, scattering all wavelengths almost equally.
Example: Fog or thick smoke appearing white/grey.
Role of wavelength in Scattering of light: Shorter wavelengths (blue colour) scatter more strongly than
longer wavelengths (red colour) in case of the Rayleigh scattering
Role of Scattering in Daily Life-
This phenomenon explains many natural events, such as the blue color of the sky, the reddening of
the Sun at sunrise and sunset, and the white appearance of clouds.
Phenomena involved on the Atmospheric Refraction
1-Blue Colour of Skyi.
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The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of
visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light
of longer wavelengths at the red end.
The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes
through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more
strongly than red and scattered blue light enters our eyes and sky appears blue.
2- The danger signal or sign is made of red colour:
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As red colour has the longer wavelength so red colour is least scattered by fog or smoke or atmospheric
particles and reaches our eyes.
Hence red colour can be seen of same colour (red colour) clearly from the long distances also.
3-Sky appears dark from space and high altitude in the atmosphere:
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There is no atmosphere in the space so there will be no scattering of light.
The sky appears dark to passengers flying in plane at very high altitudes, as scattering is not prominent at
such heights due to negligible atmosphere in high altitude
4-Sky appears dark from Moon:
There is no atmosphere around the moon so there will be no scattering of over the Moon.
5-Colour of Sun:
Colour of Sun is white at Noon:
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At noon, the sun is overhead and sunlight would travel shorter distance relatively through the atmosphere.
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Hence the sun appears white at noon because only the little of the blue and violet colours are scattered.
The Colour of Sunrise and Sunset is reddish:
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While sunset and sunrise, the colour of the sun and its surrounding appear red.
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During sunset and sunrise, the sun is near to horizon, and therefore, the sunlight has to travel larger
distance in atmosphere by which most of the blue light (shorter wavelength) is scattered away by the
particles.
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The light of longer wavelength (red colour) scattered least reaches our eye. This is why sun appear red in
colour.
Tyndall Effect is an example of Scattering of light:
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The earth’s atmosphere consists of suspended particles of dust, smoke, tiny water droplets and
molecules of air of colloidal dimension (1 to 1000 nanometers in size).
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When a beam of light passes through the colloidal solutions, the path of beam become visible due to
scattering of light by colloidal particles and this phenomenon is called as Tyndall Effect.
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It is also observed when sunlight passes through a canopy of a dense forest.