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Author Topic: How does a prism work to split up sunlight?  (Read 17857 times)

Offline chris

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How does a prism work to split up sunlight?
« on: 06/12/2010 08:11:47 »
When a beam of white sunlight enters a prism it is split up into a spectrum, a bit like a rainbow.

Why does this happen?


 

Offline JP

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How does a prism work to split up sunlight?
« Reply #1 on: 06/12/2010 09:30:25 »
When a ray of light goes from one material to another, it bends.  The amount of bending depends on the angle at which it strikes the surface, as well as on the difference in the speed of light in the two materials.  The bigger the speed difference, the more it bends.

Prisms work because the speed of light in glass depends on the wavelength (i.e. the color) of the light.  Therefore if you have a bunch of rays of different colors coming from the same direction and striking the surface, the amount they bend depends only on the relative speed of light between the prism glass and air.  Since different colors bend different amounts, you get different colors coming out of the prism in different directions.

Of course, light is a wave, not a ray, but this model accurately describes what's going on in the more rigorous and complicated wave theory.
 

Offline lightarrow

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How does a prism work to split up sunlight?
« Reply #2 on: 06/12/2010 12:22:44 »
Just to add a bit of unrelated information: the fact different colours are bent by glass at different angles, makes glass not the ideal choice for optical instruments, for example; infact a bright point of the source is not focused in a precise point of the eye/instrument, but smeared in different points, each colour in a different point (chromatic aberration). Other materials are better, in the sense that they "disperse" less the various colours; technically the material refractive's index vary less with light frequency. One of the best materials in this sense is Fluorite, crystalline calcium fluoride CaF2. The so called "apochromatic" objectives of refractory telescopes are made of it. (Not mine, because I'm not so rich  [:-'(])
« Last Edit: 06/12/2010 12:25:41 by lightarrow »
 

Offline Bored chemist

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How does a prism work to split up sunlight?
« Reply #3 on: 06/12/2010 19:30:15 »
The (even less relevant) clever trick is to get two lenses with the same dispersion, but different strengths so most of the chromatic aberration is cancelled out , but the lens still works.
http://en.wikipedia.org/wiki/Achromatic_lens

Incidentally, the reason that a beam of light bends as it moves from one material to another is that light travels at different speeds through the different materials.
It would also work with, for example, a column of soldiers marching through a desert who cross a wide road. There's nothing magic about light waves needed to explain it.
 

Offline CZARCAR

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How does a prism work to split up sunlight?
« Reply #4 on: 06/12/2010 19:42:58 »
whoa! i got double panes on my windows & no rainbow. Aint a prism pyramid shaped?
 

Offline Geezer

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How does a prism work to split up sunlight?
« Reply #5 on: 06/12/2010 19:54:31 »
whoa! i got double panes on my windows & no rainbow. Aint a prism pyramid shaped?

A prism can be pyramid shaped, but any piece of glass with surfaces that are not parallel will demonstrate the effect. Glass with bevelled edges is one example.

The reason you don't see a rainbow spectrum with your double glazing is because the sheets of glass have parallel surfaces, so the bending effect gets cancelled out when the light emerges on the other side of the glass. At least, I think that's the reason.

Lightarrow, perhaps you could use a reflector instead. They are not too expensive.
 

Offline JP

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How does a prism work to split up sunlight?
« Reply #6 on: 07/12/2010 04:03:48 »
whoa! i got double panes on my windows & no rainbow. Aint a prism pyramid shaped?

A prism can be pyramid shaped, but any piece of glass with surfaces that are not parallel will demonstrate the effect. Glass with bevelled edges is one example.

The reason you don't see a rainbow spectrum with your double glazing is because the sheets of glass have parallel surfaces, so the bending effect gets cancelled out when the light emerges on the other side of the glass. At least, I think that's the reason.

Good point!  I missed that in my explanation.  All the light strikes the front of the prism at the same angle, but when it hits the back, the different colors, which are now traveling in different directions, strike at different angles, which means they come out at different angles, which separates them.

I guess in a window pane, you could send in a narrow beam of white light and you'd see it separate...  All the colors would come out in the same direction, but they'd be shifted laterally, I think.
 

Offline techmind

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How does a prism work to split up sunlight?
« Reply #7 on: 07/12/2010 22:05:14 »
When a beam of white sunlight enters a prism it is split up into a spectrum, a bit like a rainbow.

Why does this happen?

You know about Snell's Law of refraction, which says how light bends when crossing from material of one refractive index to another. Most materials exhibit "dispersion", which means that the refractive index is slightly different for different colours, so they get bent different amounts.
With parallel-sided glass, the "bending" as the light enters the glass is symetrically "unbended" when it comes out the other side (although there'll be a very small lateral displacement of colours). With non-parallel sided glass -including cut diamonds- the light is non "unbended" equally and the colours leave the glass with a net spread of directions.
 

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How does a prism work to split up sunlight?
« Reply #7 on: 07/12/2010 22:05:14 »

 

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