How do color filters work? : Dot Physics

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Rhett Allain is an Associate Professor of Physics at Southeastern Louisiana University. He enjoys teaching and talking about physics. Sometimes he takes things apart and can't put them back together.

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How do color filters work?

Category: Demo • color • for kids • laser • light • video
Posted on: May 4, 2010 11:01 AM, by Rhett Allain

So you have seen these color filters (or gels as they are also called). When you look through a red filter, everything looks red. What do they do to the light? I am not going to tell you the answer. However, I will show you some examples so that you can figure out the answer yourself.

In this video, I am going to use a red and a green laser pointer. The nice thing about laser pointers is that they essentially create only one color of light.

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When I read the above-the-cut paragraph, I assumed you were going to do something hand-wavey which relied on approximately unimodal filters. But good use of lasers.

Posted by: rpenner | May 4, 2010 1:01 PM

It's been a while since I took physics, but from what I recall... those gels each absorb certain wavelengths of light. Sometimes that can be a very narrow band (such as with the red and green gels) or it could be a wider range of wavelengths (magenta).

If the gel doesn't absorb the photons, then the light passes through. The red gel allows the reddish wavelengths through, so that's all get we can see through it. Green light is different enough in length that it can't pass through.

As for how the gel can be so selective... I'm not sure, but I'd be interested in a more detailed description as to what's going on at the atomic level!

Posted by: RyanR | May 4, 2010 2:13 PM

Lots of additional questions for you...

So a red gel passes red light, easy. But why does a red gel appear red? Is that all from light passing through it? What is the surface color?

On the molecular level, do you happen to know what atoms or bonds are responsible for the absorption in these materials? Do they re-radiate and/or reflect? Is all the re-radiation infra/heat, or are there say, blue filters which radiate red when illuminated with green light?

Posted by: travc | May 5, 2010 4:45 AM

@travc

Why does a red gel appear red? Good questions - I guess it also reflects some red light and when it is back light, only red light passes through it.

I am not certain exactly of the atomic-level mechanism that makes this work.

Posted by: Rhett Allain | May 5, 2010 9:04 AM

"But why does a red gel appear red? Is that all from light passing through it? What is the surface color?"

I'll take a shot at answering these. A red gel appears red only because light passes through it. If you were to place one on a surface where no light passed through, such as a black piece of paper, it would just appear black.

Posted by: Paul | May 5, 2010 9:06 AM

The red gel appears red since it is absorbing other wavelengths in the visible spectrum (except red), where the red wavelength is scattered and reflected back to the viewer's eye. The green gel is the same way. The blue gel is absorbing all but the high energy wavelengths (blue-ultraviolet), thus neither the red nor the green laser will pass through.

The gel is probably scattering a portion of the laser light and absorbing the rest of it, returning to ground state non-radiatively. Cool demo, I wish I had this when TAing Gen Chem.

Posted by: Mike | May 5, 2010 1:28 PM

what do coulor filters do to colors?

Posted by: jaedon and jamal experiment | March 4, 2011 4:48 PM

what do color filters do to colours because me and my best friend are doing a science experiment

Posted by: jamal | March 4, 2011 5:06 PM

The critical concept here is not absorption, as someone before tried to explain, but transmittance and interference ;)

Posted by: Alex | April 13, 2011 9:56 AM

I thought this was really good!

Posted by: Taymar! | May 5, 2011 3:22 PM

please ans me how interference is used in colour filters

Posted by: nancy | August 21, 2011 11:36 PM

When the red light passes through the green filter no light comes out i see. but what im wondering is why? why does o light come out?