How Everything Works
How Everything Works How Everything Works

Question 636

Why is the sky blue? - Z
As it passes through the atmosphere, sunlight can be deflected by a process known as Rayleigh scattering. When sunlight passes through any material, its light waves cause electric charges in the material to jiggle back and forth. That's because light waves contain electric fields and electric fields exert forces on electric charges. When the charges in a material jiggle back and forth, they may emit light. In this case, the material can absorb the sunlight for an instant and reemit it in a new direction. This process, whereby jiggling electric charges in a material absorb a light wave and reemit it in a new direction, is Rayleigh scattering.

Rayleigh scattering is extremely inefficient in particles that are much smaller than the wavelength of the light, so that visible light can travel through miles of molecules in the atmosphere before it experiences significant Rayleigh scattering. But blue light has a shorter wavelength than red light and thus experiences Rayleigh scattering more often than red light. As a result, the atmosphere tends to send the blue portion of sunlight off in every direction. Thus when you look at the atmosphere, it appears blue.

A reader (TAC) points out that the above explanation would seem to imply that the sky should appear violet, since violet light scatters more strongly than blue light. But the spectrum of sunlight peaks in the green—sunlight contains more green light than blue light and more blue light than violet light. The sky combines these two effects together (more green light but better scattering of violet light) and acquires an overall blue appearance.


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