At the moment I'm somewhat confused by the concept of colored filters; common sense states that they allow only their color of light to pass through(i.e. red filter lets red light through), but, if they appear to be a specific color, wouldn't that indicate that they reflect that color?

  • $\begingroup$ Take a look at some dichroic filters. $\endgroup$ – PM 2Ring Feb 28 at 10:51
  • $\begingroup$ Also consider the material can be supported by a substrate. This can even change the colour of something depending on the side of viewing (like obscured glasses). I had prepared myself a thin film on glass with two different transmission spectra. It doesn't happen so often but it should be a right combination of refraction index at the relevant wavelengths. $\endgroup$ – Alchimista Mar 1 at 9:46

You're right, an ideal filter can either absorb or reflect the spectrum it should not pass, and thus for that ideal filter we should not see that pass-band color reflected from the filter. Unfortunately, practical materials are less efficient and typically reflect a little of the spectrum they are supposed to pass (the degree of inefficiency depends on the type and spectrum of filter). The reason you often see a filter that has a reflection color the same as the color it is suppose to pass, is that it absorbs all other colors (so even the little that gets reflected from some internal structures quickly gets absorbed before it makes it out of the material) but the little pass-band color that does reflect from internal structures does make it out of the material for us to see, because that color isn't absorbed much. On top of that, the eye-brain system causes us to consider something clearly colored (we say its "red") even though it may barely have more red than other colors.


I would expect that the highest quality filters do not reflect any of their colour (red for example) and only appear red because everything behind them appears red. As it blocks out all the other wavelength's of visible light. A red filter like that would appear black in a completely blue room, however it is not actually completely black, the same would apply to a blue filter in a red room.

This would be different for filters that are likely to be used in a school classroom where the light filters do indeed reflect a small portion of their respective light and a red filter is genuinely red (as it appears).

Hope this helps!:-)

  • $\begingroup$ But how do you see the red? Saying " only appear red because everything behind them appears red" isn't like to see that red light is reflected to us? Like in the other answer? $\endgroup$ – Alchimista Mar 1 at 9:50
  • $\begingroup$ @Alchimista Sorry, I don't really understand your question. What I am saying is that the filter doesn't necessarily reflect red onto our retina from an existing light source, but rather only allows the already reflected red from the objects behind it to pass through and into the eye. Hence our brain only sees the colour of red through the filter and thus associates the colour of the filter with the one behind it. I'll use the same example again to say that the same red filter would appear black in a blue room, but that doesn't mean that it absorbs all the wavelengths of light. $\endgroup$ – steve_just_steve Mar 1 at 10:19
  • $\begingroup$ Ok if seen by transmission I would not call it reflection. But it is true. Got it. It is actually in the question that one has to be careful as for the supposed reflection might be indeed transmitted light instead, as it should be. $\endgroup$ – Alchimista Mar 1 at 10:55

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