# Does Color Filter Change The Frequency Of Light?

quantum mechanics electromagnetic spectrum

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### #1 vibhorsethi

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Posted 05 September 2018 - 01:02 AM

If I glow a white LED bulb and then put a color filter around it (for example, a red color filter or a violet color filter) , then will it change the frequency of light.

### #2 exchemist

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Posted 05 September 2018 - 02:50 AM

If I glow a white LED bulb and then put a color filter around it (for example, a red color filter or a violet color filter) , then will it change the frequency of light.

No.

The clue is in the word "filter". If you filter coffee, does the filter change the coffee? No, it just selectively removes the grounds.

A light filter absorbs many frequencies and thus transmits a narrower range of them than is present in the incident light. If you have white light and a red filter, the filter absorbs all the frequencies higher than those that give red light, i.e. absorbs frequencies > 484THz approximately.

See here for frequency and wavelength ranges of visible light:  https://en.wikipedia...isible_spectrum

### #3 hazelm

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Posted 02 December 2018 - 11:43 AM

No.

The clue is in the word "filter". If you filter coffee, does the filter change the coffee? No, it just selectively removes the grounds.

A light filter absorbs many frequencies and thus transmits a narrower range of them than is present in the incident light. If you have white light and a red filter, the filter absorbs all the frequencies higher than those that give red light, i.e. absorbs frequencies > 484THz approximately.

See here for frequency and wavelength ranges of visible light:  https://en.wikipedia...isible_spectrum

I don't understand, exchemist.  (Quote from Wiki:  "The frequency of the wave determines its color:4×1014 Hz is red light,8×1014 Hz is violet light, and between these (in the range 4-8×1014 Hz) are all the other colors of the visible spectrum.")  So why wouldn't changing the color from white to red change its frequency?  Does white not have a frequency?  White and black - I always forget which is the presence of all colors and which is the absence of all colors.  But does that have something to do with it?  And, yes, I did read the Wiki article but the question remains.

### #4 exchemist

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Posted 02 December 2018 - 12:14 PM

I don't understand, exchemist.  (Quote from Wiki:  "The frequency of the wave determines its color:4×1014 Hz is red light,8×1014 Hz is violet light, and between these (in the range 4-8×1014 Hz) are all the other colors of the visible spectrum.")  So why wouldn't changing the color from white to red change its frequency?  Does white not have a frequency?  White and black - I always forget which is the presence of all colors and which is the absence of all colors.  But does that have something to do with it?  And, yes, I did read the Wiki article but the question remains.

Hazel, you ask the right question. No, white light does not have a frequency - or rather it has a whole set of them, all the way from red to violet.

White is hotchpotch of all the visible frequencies. So when you put a red filter in front of a white light, it absorbs the frequencies corresponding to all the other colours, e.g. violet, blue, green, yellow etc, and only transmits the frequencies corresponding to red.

That's why it is called a "filter", rather than a "colour-changer" or something.  A "filter" allows something in a mixture to pass through, while holding back the rest, just as a coffee filter keeps back the grounds and allows only the liquid through.

### #5 hazelm

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Posted 02 December 2018 - 12:24 PM

Hazel, you ask the right question. No, white light does not have a frequency - or rather it has a whole set of them, all the way from red to violet.

White is hotchpotch of all the visible frequencies. So when you put a red filter in front of a white light, it absorbs the frequencies corresponding to all the other colours, e.g. violet, blue, green, yellow etc, and only transmits the frequencies corresponding to red.

That's why it is called a "filter", rather than a "colour-changer" or something.  A "filter" allows something in a mixture to pass through, while holding back the rest, just as a coffee filter keeps back the grounds and allows only the liquid through.

So the red you ended up with was already there at red's frequency.  And (answering my own next question)  all the other frequencies are still there in the white but the viewer no longer sees them  Am I right?  Meaning that the filter isn't changing the frequencies.   It is changing which one we will see.  Yes?

Thank you.

Edited by hazelm, 02 December 2018 - 12:25 PM.

### #6 exchemist

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Posted 04 December 2018 - 02:54 AM

So the red you ended up with was already there at red's frequency.  And (answering my own next question)  all the other frequencies are still there in the white but the viewer no longer sees them  Am I right?  Meaning that the filter isn't changing the frequencies.   It is changing which one we will see.  Yes?

Thank you.

Yes the red was there in the white, along with the yellow, green blue etc etc. And yes, the filter is NOT changing the frequencies. It is subtracting all of them (by absorbing them)  except the red. A filter will in fact get warm, due the absorption of all the light it is not allowing to pass.

### #7 hazelm

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Posted 04 December 2018 - 05:58 AM

Yes the red was there in the white, along with the yellow, green blue etc etc. And yes, the filter is NOT changing the frequencies. It is subtracting all of them (by absorbing them)  except the red. A filter will in fact get warm, due the absorption of all the light it is not allowing to pass.

Great!  Quite clear.  Thanks.