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If photons encounter particles whose size is similar or smaller than their wavelengths, they can get reflected off to either side depending on phase of their vibration at the encounter. If the particles are less than <10 nanometers, their scattered direction becomes random (Rayleigh scattering) – some photons may even barrel on through. The chance of such encounters increases with photon vibration frequency. Higher frequency vibrations are more likely to sweep them into such particles, while lower frequencies are more likely to let them slip past. Put another way, more short wavelength photons are scattered than longer ones.

Were it not for this scatter, on a clear day the sky would look black every where except when you looked directly at the sun. (DON’T EVER.) However, clusters of water molecules from 0.3 to 100 nm suspended in the atmosphere are the right size to scatter photons. Therefore rays of sunlight original headed elsewhere will have some of their photons scattered in your direction. Since more short wavelength photons are scattered, they predominate. Their blue sensory effect is attributed to the sky in the direction from which they arrived.

The water droplets that make up clouds are several orders of magnitude larger. Therefore clouds are Lambertian, reflect all wavelengths equally, and appear white. A continuous range of particle sizes from single atoms of water to rain drops provide a range of effects depending on conditions.


Glance at the Dance of Photons Copyright © 2022 by Thomy Nilsson. All Rights Reserved.

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