the sky

[mrutley]

why is the sky blue

[WaldoMcFroog]

A very short answer is that light is composed of many colours (in fact, white light as any Pink Floyd fan will know) can be split via a prism into the full spectrum. Different colours inhabit different parts of the visible spectrum - that is that their frequencies are slightly different. Certain frequencies will be refracted (effectively bent away from hitting your eyes) by passing them through particular materials. For instance, if you hold a piece of red cellophane in front of your eyes, you will see that blues and yellows all but disappear - they become red (or red tinged). The sky is the same - on a clear sunny day, the gasses in the atmosphere refract many of the frequencies of the sun's rays, leaving predominantly blue.

[incitatus]

The great waldo is correct...the sky is blue due to refraction. However red cellophane transmits red light due to absorption, which is different.... Yours stirringly...

[Hamish]

I'm sorry folks but this isn't right.... If the sky was blue due to refraction then the angle you looked at the sky from would affect the colour you saw, also if only blue light is reaching us then the lovely prisum experiment you describes wouldn't work because only blue light would be preasent, also rainbow would only consist of blue. The reason the sky is blue is due to the scattering of light. Particles (ice, small molecules etc) scatter the light at low wavelengths so that the blue end of the spectrun passes stright through. This way all wavelengths get to earth (hence rainbows) but blue is seen as the predominant colour. The reason redmist can sometimes be seen over a city is the same but in this case the the red light isn't scattered. The effect can also be seen in liquids where a very fine dispersion is preasent. Hope this helps, Hamish

[WaldoMcFroog]

But the scattering is surely just refraction? I'm sure I recall reading in one of the excellent books by phyiscist Richard Feynmann about refraction gratings, and he made the point that this was what was happening with a rainbow - the interfacing of sunlight via rain refracts the light in different ways - infact acting as a prism. I'm entirely open to correction of this though!

[Hamish]

Refraction occours as light passes through a transparent material wich has a different refractive index to the material arround it. I have never heard of a refraction grating, but there is such a thing as a diffraction grating. Here there is a slit that is less than half of the wavelength of the light you wish to diffract. The effect with particals is similar only each partical is involved in diffrection and as there are millions of particals the light is scattered. I think you may have misunderstood Feynmann. You are correct however that Refraction is responsible for the rainbows, here light passes through the water droplets and the light is refacted and split into the full spectrum, you then see all the colours not just one (Blue). Hope this is clearer now, if you want more explanation just say. All the best Hamish.