Human Eye and Colourful World

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Spreading of light in various directions by colloid particles.

While sunset and sunrise, the colour of the sun and its surrounding appear red. During sunset and sunrise, the sun is near horizon and therefore the sunlight has to travel larger distance in the atmosphere.

Due to this most of the blue light (shorter wavelength) are scattered away by the particles. The light of longer wavelength (red colour) will reach our eye. This is why the sun appears red in colour.

Tyndall effect : When light passes through a colloid its path becomes visible. This is called Tyndall effect. E.g.,

1. Path of light becomes visible when light enters a dark and dusty room through a slit or ventilator.
2. Path of light becomes visible when light passes through the dense canopy of trees in a forest.

The upper layer of the atmosphere contains very fine particles of water vapours and gases. These particles are more effective in the scattering of light of shorter wavelength mainly blue than the larger wavelength. So, the sky appears blue.