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# Why do the constituent of white colour have different wavelength? Answered

When white light passes through a prism,it is splitted into its constituents colours due to the difference in the speed of different colours in prism.the colour which has more speed is deviated most(red)and the colour having less speed deviates least(violet).thus,white light is splitted.
we know that wavelength is directly proprtional to speed. The question is why the wavelength of different colours differ from each other?and why does the wavelength of red colour is longest and of violet colour is shortest?

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## 14 Replies

kelseymh (author)2010-10-09

"We know that wavelength is directly proportional to speed." That's not correct.

The fact that red is long wavelength and violet is short is just definition. The word names are mapped to the wavelengths based on our perception.

If you're asking why they get bent by different amounts in passing from air to glass, that is because the index of refraction of glass is not a constant, but is rather a function of wavelength (or better, a function of frequency). In particular, the index of refraction is higher for violet (so it is slowed down more in glass, and gets bent more), and not so high for red (so it is not slowed down as much, and gets bent less).

The general rule is that the index of refraction increases with frequency, as in glass, water, diamond, etc., but that is not universally true.

Kiteman (author)2010-10-09

It's not "speed" which splits light as it passes through a prism, but the angle at which they are refracted.

That angle depends on the wavelength.

kelseymh (author)2010-10-09

It's both. The index of refraction is just the ratio of speeds of light in vs. out of the medium. Since that index is wavelength dependent, you get different bend angles as a function of wavelength.

steveastrouk (author)2010-10-09

Actually it IS speed....

Kiteman (author)2010-10-09

>doh<

kelseymh (author)2010-10-09

I'm a bit confused here, Steve. If N is independent of wavelength, then you get the same bend angle for all wavelengths, and consequently no spread (dispersion) of the output beam. The only reason a prism can make a "rainbow" at all is dispersion. What am I missing?

orksecurity (author)2010-10-09

Note that wavelength is only directly proportional to speed when frequency stays constant. The frequencies of different colors of light are different.

steveastrouk (author)2010-10-09

Except speed here is constant, axiomatically

orksecurity (author)2010-10-09

That's my point; the assertion that the two values are proportional made an assumption which isn't true.

seandogue (author)2010-10-09

Erm radio waves are light, just not in the region visible by humans

steveastrouk (author)2010-10-09

Isn't that the exact point I was making ?

seandogue (author)2010-10-09

I'm sorry, what source? Do you mean the sun? The incandescent light bulb in your kitchen? The flourescent bulb?

After all, it's not magic, regardless of any personal desire for it to be.

The "white" light is not "white" at all. instead that is our perception as the constituent wavelengths from the "source" are dithered by our eyes and brain.

God, but I wish people would stop being so damned mystical and trying to make science into magic.

rickharris (author)2010-10-09

Ah - Octarine!

orksecurity (author)2010-10-09

There was a recent question here on Answers where we went into a fair amount of detail on this (correcting my failed memory of freshman physics in the process)...