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1) I understand light dispersion. But, I wonder what happens if I used a lens to focus it into a small point. Would it be a white point (would it reverse dispersion) or would I have a multicolored point?

2) I know how do light filters work. Like red filter only lets red light through. But are there any filters that allow (for example) only light with wavelength shorter than 600nm? I mean, not only that one color, but everything "shorter".

3) And last, Are color filters (red, blue, etc) only solid or liquid objects? Is it possible to use gas as a filter? Or even something "smaller"?

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Please just ask one question at a time. –  EnergyNumbers Nov 10 '12 at 8:57
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1 Answer

up vote 1 down vote accepted

First: I'd guess that by light dispersion you mean chromatic aberration. In general refocussing the light will not undo chromatic aberration, but it's possible to specifically design pairs of lenses to (mostly) eliminate it. These are called achromatic doublets.

Second: yes, you need a bandpass filter. It's not possible to transmit everything below 600nm because the glass of the lens will start absorbing in the ultraviolet and shorter. However you can design filters to transmit over a range e.g. 600nm to 400nm. Google for optical bandpass filter to find lots of suppliers selling bandpass filters.

Last: in principle anything will work as a filter, though the range of wavelengths transmitted may not be convenient. For example carbon dioxide filters out infra-red light, which is how it causes global warming. There are a few coloured gases e.g. chlorine, bromine and iodine, but the range of colours available is limited compared to what can be achieved in a solution or solid filter.

Response to comment:

Re the first question, in principle I think you can refocus the light from a prism to a point, or at least to some arbitrarily small area, But I don't know offhand how to do it.

Re the second question (which I can answer :-): the sort of filters used in cameras typically rely either on the optical absorption of some chemical or on a fixed interference system and they can't be changed. When you need a tunable filter you would normally use a diffraction grating. The amount the light is diffracted depends on it's wavelength, so the originally white light is split up into the different wavelenghts. By adjusting the angle of the diffraction grating you can control what wavelength light falls on your experiment. If you've ever used a spectrometer at college this is almost certainly how the spectrometer adjusted the wavelength.

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First of all, thank you for nice, fast answer. But I've got 2 more questions. 1. Sorry, I was unclear. I want to let light through prism, then use lens to focus it. Will you answer still be the same? 2. Is there any filter that has will change it's "filter ratio" (or whatever it's called). I mean, is there a way to control filter, to make it filter out wavelenghts I want in this very moment? Like, infra-red, but then I want to change to green (was it like 550 nm?), and then to UV. Is this possible? Ofc better yet if it could be controlled via software. –  Dreat Nov 9 '12 at 20:16
    
@Dreat this isn't a discussion forum, it's a Q&A site. Just post one new question for each question you've got, as a new question - not as a supplementary question-within-a-question, nor as a comment. When you ask each question, please do mention what research you've done on it to date, so we know which is the bit you're unsure about. –  EnergyNumbers Nov 10 '12 at 8:59
    
@Dreat: I've edited my answer to respond to your questions. As EnergyNumbers says, if you'd like a more detailed answer please post a new question. –  John Rennie Nov 10 '12 at 10:06
    
Thank you very much, and sorry. Will post one question only next time. My bad. –  Dreat Nov 10 '12 at 21:42
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