Scattering of Light

 Scattering of Light is the phenomenon of deflection of light when the light wave strikes to particles or molecules of solid, liquid or gas comparable to the size of wavelength of light. If the particles are relatively large, reflection or refraction as well as diffraction may take part. If the particles are relatively small the effect is almost diffractive. The blue color of sky is due to scattering by air molecules and the red sun is due to removal of blue light from by the scattering from the red beam.

Q. Why day sky looks blue?

The blue component of the spectrum of visible light has shorter wavelengths and higher frequencies than the red component. Thus, as sunlight of all colors passes through air, the blue part causes charged particles to oscillate faster than does the red part. The faster the oscillation, the more scattered light is produced, so blue is scattered more strongly than red. For particles such as air molecules that are much smaller than the wavelengths of visible light the difference is dramatic. The acceleration of the charged particles is proportional to the square of the frequency, and the intensity of scattered light is proportional to the square of this acceleration. Scattered light intensity is therefore proportional to the fourth power of frequency. The result is that blue light is scattered into other directions almost 10 times as efficiently as red light.

When we look at an arbitrary point in the sky, away from the sun, we see only the light that was redirected by the atmosphere into our line of sight. Because that occurs much more often for blue light than for red, the sky appears blue. Violet light is actually scattered even a bit more strongly than blue. More of the sunlight entering the atmosphere is blue than violet, however, and our eyes are somewhat more sensitive to blue light than to violet light, so the sky appears blue.

 Q. How sun looks reddish morning and evining?

The red wavelengths of sunlight that pass through the atmosphere without being scattered much reach our eyes, while the strongly scattered blue light does not. The longer distance that the sunlight travels through the atmosphere when it is on the horizon amplifies the effect--there are more opportunities for blue light to be scattered than when the sun is overhead. Thus, the setting sun appears reddish.