Scattering of light

• Scattering: Scattering of light is throwing light in various random directions.
• Light is scattered when it falls on various types of suspended particles in its path.

TYNDALL EFFECT

• The scattering of light by particles in its path is called Tyndall effect.
• Tyndall effect can also be observed when sunlight passes through the canopy of a dense forest.
• Tyndall discovered that when white light consisting of seven colours is passed through a clear liquid having small suspended particles in it, then the blue colour of white light having shorter wavelength is scattered much more than the red colour having longer wavelength.

COLOUR OF SCATTERED LIGHT

The colour of the scattered light depends on the size of the scattering particles in the atmosphere:

• The larger particles of dust and water droplets present in the atmosphere scatter the light as much due to which the scattered light also appears white.
• The extremely minute particles such as the air molecules present in the atmosphere scatter mainly the blue light present in the white sunlight.

WHY IS THE SKY BLUE?

• The scattering of blue component of the white sunlight by air molecules present in the atmosphere causes the blue colour of the sky.
• The sunlight is made up of seven coloured lights mixed together.
• 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.

In outer space, the sky looks dark and black instead of blue: This is because there is not atmosphere containing air in the outer space to scatter sunlight. Scattering is not prominent at such heights.

WHY THE SUN APPEARS RED AT SUNRISE AND SUNSET?

• Light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere before reaching our eyes.
• Light from the Sun overhead travels relatively shorter distance. So, at noon, the Sun appears white as only a little of the blue and violet colours are scattered.
• 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.
• At the time of sunrise and sunset when the sun is near the horizon, the sunlight has to travel the greatest distance through the atmosphere to reach us. During this long journey of sunlight, most of the shorter wavelength blue-colour and shorter wavelengths present in it is scattered out. Therefore, the light that reaches us is of longer wavelengths. Hence the sun appears red.