Human Eye and Colourful World: Atmospheric Refraction
Deviation of ray of light when it passes through different layers of atmosphere is called atmospheric refraction.
Different layers of atmosphere have different optical densities. Due to this, ray of light deviates from its path when it passes through subsequent layers of atmosphere. Many interesting phenomenon happen because of atmospheric refraction.
You have already read that when light enters from one medium to another, there is a deviation in its path. This phenomenon is called refraction of light.
These notes are based on the chapter Human Eye and Colourful World from class 10 science NCERT book and CBSE syllabus.
- Twinkling of Stars
- Advanced sunrise and delayed sunset
- Scattering of light
- Tyndall effect
Twinkling of stars
The twinkling of a star is due to atmospheric refraction of starlight. On entering the earth's atmosphere, the starlight undergoes refraction continuously before it reaches the earth.
The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position. The star appears slightly higher (above) than its actual position when viewed near the horizon.
Further, this apparent position of the star is not stationary, but keeps on changing slightly, since the physical conditions of the earth's atmosphere are not stationary. Since the stars are very distant, they approximate point-sized sources of light.
As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of starlight entering the eye flickers, i.e. the star sometimes appears brighter, and at some other time, fainter, which gives the twinkling effect.
Advance sunrise and delayed sunset
The Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset. It happens because the sun appears higher than its actual position over the horizon. It happens because of atmospheric refraction.
By actual sunrise, we mean the actual crossing of the horizon by the Sun. The time difference between actual sunset and the apparent sunset is about 2 minutes; and same holds true for actual and apparent sunrise.
Rays of light from the sun bend towards normal when they enter a layer of higher optical density from another layer of lower optical density. Due to subsequent bending through further layers of atmosphere, the sunlight appears to be coming from a higher position than the actual position of the sun over the horizon.
The apparent flattening of the Sun's disc at sunrise and sunset is also due to the same phenomenon, i.e. atmospheric refraction.
Scattering Of Light
When light hits a particle, it scatters in different directions. Scattering of light happens because of non-uniformities of particles of a medium. Many interesting phenomenon can be observed because of scattering of light. Some of them are given here.
Tyndall Effect
The optical effect because of scattering of light from the particles of colloid or suspension is called Tyndall Effect. For Tyndall effect to be possible, the size of particles should be less than or equal to the wavelength of the visible spectrum. So, the size of particles should be between 40 and 900 nanometer.
Tyndall effect is responsible for many natural phenomena. The white beam of light which appears to come through the ventilation or through a slit in the door is because of Tyndall Effect and the dust particles in the air cause the scattering of light in this case. The white beam appears because scattering of light makes the dust particles visible in the light.
Why is the colour of the clear Sky Blue?
We know that the wavelength of red colour is more than that of blue colour. The size of particles in air is smaller than the wavelength of visible light. Hence, these particles scatter the light of shorter wavelength more effectively than light of longer wavelength.
The blue end of the visible spectrum has shorter wavelength than the red end. Due to this, blue colour is scattered more strongly in the atmosphere; compared to the red colour. This is the reason sky appears blue. Since red colour is scattered the least hence it is used in traffic lights for showing the danger signal.
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