Page 174 - NCERT Science Class 10 English Medium

10.6 SCATTERING OF LIGHT

The interplay of light with objects around us gives rise to several
spectacular phenomena in nature. The blue colour of the sky, colour of
water in deep sea, the reddening of the sun at sunrise and the sunset
are some of the wonderful phenomena we are familiar with. In the previous
class, you have learnt about the scattering of light by colloidal particles.
The path of a beam of light passing through a true solution is not visible.
However, its path becomes visible through a colloidal solution where the
size of the particles is relatively larger.

10.6.1 Tyndall Effect

The earth’s atmosphere is a heterogeneous mixture of minute particles.
These particles include smoke, tiny water droplets, suspended particles
of dust and molecules of air. When a beam of light strikes such fine
particles, the path of the beam becomes visible. The light reaches us,
after being reflected diffusely by these particles. The phenomenon of
scattering of light by the colloidal particles gives rise to Tyndall effect
which you have studied in Class IX. This phenomenon is seen when a
fine beam of sunlight enters a smoke-filled room through a small hole.
Thus, scattering of light makes the particles visible. Tyndall effect can
also be observed when sunlight passes through a canopy of a dense
forest. Here, tiny water droplets in the mist scatter light.
The colour of the scattered light depends on the size of the scattering
particles. Very fine particles scatter mainly blue light while particles of
larger size scatter light of longer wavelengths. If the size of the scattering
particles is large enough, then, the scattered light may even appear
white.

10.6.2 Why is the colour of the clear Sky Blue?

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. The scattered blue light enters
our eyes. If the earth had no atmosphere, there would not have been
any scattering. Then, the sky would have looked dark. The sky appears
dark to passengers flying at very high altitudes, as scattering is not
prominent at such heights.
You might have observed that ‘danger’ signal lights are red in colour.
Do you know why? The red is least scattered by fog or smoke. Therefore,
it can be seen in the same colour at a distance.

The Human Eye and the Colourful World 169

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