Dear Jaime Lasdauskus

I'm sure you've heard of radiation. Well, not all radiation is bad or dangerous like the radiation from nuclear reactors or bombs. Most radiation is good for us--in the right doses. We couldn't live without it.

Heat radiation is electromagnetic radiation with a medium wavelength.

The wavelength is like the distance between ripples that are travelling over a pond.

But what's this got to do with the sky's being blue?

What happens is that the light from the sun is white. Now you may know that white light is really a mixture of different colours...or different wavelengths of light. When all these different wavelengths reach the top part of the atmosphere most of them pass straight through but some of them don't. That's because there are molecules of gas (the air) that are about the same size as the wavelength of the blue and violet light (about 35 micrometres or 35 millionths of a metre across). So they get bumped off course.

The ultraviolet particles are just the right size to get absorbed by ozone molecules (groups of three oxygen atoms joined together). That's why the ozone layer is important--it absorbs most of the dangerous ultraviolet radiation.

But the blue light is scattered a bit and not absorbed so much. It zips and zaps around the sky until it finally bounces in our direction. So we see some of the sun's blue light coming from all over the sky.

That's why the sky does not appear to be blue at night. There's not enough blue light from the moon or stars for us to notice.

But during day time it is different. Because visible light is most intense at 0.5 micrometres (which we see as blue), more light is scattered at this wavelength than at any other wavelength--and the sky appears blue.

Yours sincerely

Virginia R. Claire


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