Blue Sky Thinking |

“And God called the firmament Heaven. And the evening and the morning were the second day” (Genesis:1:8).

We take the sky for granted. It is just there. Yet, whether it is covered with cloud, or clear, we do not consider the fact that, during the daytime, all of the sky is somewhat light. In particular, on a clear day, the sky appears blue. The blueness itself is remarkable, given that the sun is yellow, but why is the sky any color at all?

Having seen photographs from the Moon, taken by the Apollo Astronauts, we know that even when the sun is out, the rest of the sky is black. Sunlight comes from the Sun, so the blueness of the sky on Earth must be because of light coming from elsewhere.

Of course, all the light is from the Sun originally. But that light has been scattered. So there is scattered light coming at us from many different directions when we look at the sky. The reason for this scattering of light is a phenomenon known as Raleigh scattering – named for British physicist Lord Raleigh.

Sunlight is scattered by molecules and particles in the atmosphere. It turns out that the mix of molecules and particles that we have in Earth’s atmosphere scatters light more strongly at shorter wavelengths than at longer wavelengths. Of the colors of visible light, blue has a shorter wavelength than red. So it appears to us that blue light is coming to us from all directions of the sky because that light has been scattered.

When we learn the mechanism for something, it should not spoil our wonder at it but should cause us all the more to praise our Creator.

[TBC: Another scientist notes, "Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere.  Even today, people sometimes incorrectly say that this is the case.  Later scientists realised that if this were true, there would be more variation of sky colour with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering.  The case was finally settled by Einstein in 1911, who calculated the detailed formula for the scattering of light from molecules; and this was found to be in agreement with experiment.  He was even able to use the calculation as a further verification of Avogadro's number when compared with observation.  The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules.

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