# Why shadow of light that goes through colored glass becomes colored?

The way I understand us seeing colors: for non transparent objects, light of specific wavelength (that matches the color) bounces off object (the rest is absorbed by object) and hits cones in our eye, then based on combination of cones activated (RGB) we perceive color.

For transparent things light does not get absorbed but goes through the object (nothing gets reflected, nothing hits retina) and we see through it.

For colored transparent objects all but that specific wavelength goes through, specific color gets reflected, all others go through.

Now obviously I must be making a mistake somewhere in the above...

Why when light goes through colored glass shadow becomes colored to the color of the glass? (Shouldn't it be 'White Light' minus color of the glass? e.g anything but that color)

Let's consider that transparent colored glass, which color wavelength is $\lambda$, It is colored because all the other wavelengths are absorbed and that specific wavelength $\lambda$ gets reflected, In the case when light goes through it, same thing can be observed, all the other wavelengths are absorbed and only that specific $\lambda$ gets through.

For illustration consider the image bellow:

The first filter looks red($\lambda = 700nm$) because all other wavelengths are absorbed and only red is able to go through it or get reflected, Same can be observed with the rest of the filters.

• Hi Kosta, thanks for your answer, but shouldn't absorbing all the other colors stop material being transparent? Sep 12, 2016 at 15:11
• @MatasVaitkevicius indeed, the material isn't transparent for those colors. Sep 12, 2016 at 15:13
• @MatasVaitkevicius Yes that would be the case for a perfect filter, but filters don't absorb other wavelengths fully they let some portions in. Sep 12, 2016 at 15:13
• @Ruslan Ok so my mistake is: 'Transparent' == 'does not absorb'. Sep 12, 2016 at 15:21
• @MatasVaitkevicius No, You are correct about that, Transparent for humans means that visible light doesn't get absorbed by a material, but in case of a filter some wavelengths of visible light get absorbed more(but not fully) than one specific $\lambda$ wavelength, So filter is transparent but not as much as glass which lets in all the wavelengths of visible light. Sep 12, 2016 at 15:28

For colored transparent objects all but that specific wavelength goes through, specific color gets reflected, all others go through.

If that were true, then when you were in between the light source and the colored transparent object (let's say it's red), then it would appear red, but if the colored object were between you and the light source, then it would appear cyan (=anti-red) because all light except red would pass through it. But this is not how translucent objects work in the real world.

Instead, it must be the case that they absorb all colors but one (just like a regular colored object), and that one color they partially transmit and partially reflect. (Of course they might absorb some of that color light too.) This explanation explains why you can't see perfectly through a colored translucent object (because some of the light is being reflected rather than transmitted), and also why the shadow is the same color (because the same color is the one being transmitted.)

Kosta Butbaia's answer is on the right track, but he missed the fact that quite of bit of the one color of light gets reflected as well as transmitted. His diagram suggests that if you and the light source were on the same side of the translucent object, it would appear completely black, which isn't the case for most colored transparent objects - they usually look more or less the same color on both sides, no matter where the light source is.

• The key for me to understand this was: that they absorb all colors but one (just like a regular colored object), and that one color they partially transmit and partially reflect. As long as my eye gets colored light from different points within the object I will see through it and perceive it as transparent. Thanks. Sep 13, 2016 at 9:24
• I think I mentioned that in text under the diagrams, When I said "Only red is able to go through it or get reflected" Sep 13, 2016 at 11:53
• But diagrams were a bit misleading, Because it doesn't show that fact that red light also gets reflected. But that fact is mentioned in my answer. Sep 13, 2016 at 12:02
• @Matas There are many very transparent filters such as colored glass filters that show nearly zero scattering of light. In this case, the reflected light is not colored, the only reflected light is due to specular reflection at the interface air-filter, it is not colored. Sep 12, 2021 at 21:46