How can a piece of colored plastic change the wavelength of light?

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For exaple when you put a piece of red plastic in front of a flashlight how can the wavelengths just change?

In: Physics
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They don’t – coloured plastic basically acts like a filter. Most light we interact with isnt one wavelength but a combination of various wavelengths, with white light being a relatively even distribution across the visible spectrum. If you shine white light through red cellophane all the non-red wavelengths are absorbed while the red passes unhindered, and you now have a red light.

It generally doesn’t. The colour red bounces, others don’t. That’s what makes it red. White(ish) light from a flashlight contains all the individual colours of the rainbow in it all mixed together. [A prism breaks up those mixed up colours](https://www.science-sparks.com/rainbow-with-a-prism/) – it doesn’t make colours, just separates them.

If you shine a green or blue light at a red object, it will appear black, or very nearly so since blue is mostly absorbed by a red objective. It might be a tiny bit visible but the majority of that light is absorbed, not reflected.

It doesn’t really just change. Different molecules react differently to different wavelengths of light. If you hold up a red piece of plastic all the other wavelengths are absorbed and turned to heat (it’s a very small amount of heat though usually), but the red is reflected back where it can hit your eye. So you see red.

If the plastic is red and also transparent, or something like glass, then it’ll behave similarly except that instead of causing the red light to bounce away, the red light, and only the red light, will pass through it.

A red piece of plastic is red because it reflects red light. The question is what happens to all the wavelengths of light that is not red. Depending on the plastic and the wavelength it can be absorbed or it could go straight through. So a material will end up being transparent to a completely different set of wavelength then it reflects. The only thing you can say about the light going through red plastic is that it have significantly less red in it then originally.

It should also be noted that humans have a relatively limited color vision. We can only see three different ranges of wavelengths and have to interpret the color of the light from this information. So for example a light composed of two different wavelengths may trigger the same cones as another light with a single wavelength. But these will behave differently when passing through different filters. Similarly cameras do not see the same ranges of wavelengths as humans and this is also perticularly noticable if you start filtering out single wavelengths.

Whilst keep in mind that it primarily works as a filter but a more dense medium also slows the light down which shifts the frequency towards blue. So technically yes but to such a small degree that its primary the filter doing the work, then again I might have missunderu some things but I think this is correct