Frequency of Differently Coloured Light

[Michelleeeee]

We know that light of a specific colour has a specific frequency. Suppose we have a torch emitting white light, and we place a, say, red cellophane paper, in front of it. Now we would have red light. So does this mean the new beam we get has a new frequency? How?

 

[gneill]

White light is not a single color. What you've done with the cellophane is remove all the other colors but red.

 

[jbriggs444]

We know that light of a specific colour has a specific frequency. Suppose we have a torch emitting white light, and we place a, say, red cellophane paper, in front of it. Now we would have red light. So does this mean the new beam we get has a new frequency? How?

There is a common idea that for every color there is a frequency and for every frequency there is a color. That is only true for "spectral" colors - monochromatic light. https://en.wikipedia.org/wiki/Spectral_color

Most of the colors we encounter in everyday life are mixtures of a bunch of frequencies with different amplitudes at each frequency. To fully characterize such a "color", one would have need a graph of the distribution of amplitudes across the frequency range. Human color vision does not give us such a graph. At a simplified level, we have three types of light sensing bodies in the eye. So our eyes obtain, in effect, only three numbers which we use to characterize the complete frequency distribution. https://en.wikipedia.org/wiki/Trichromacy.

There is a lot of post-processing that goes on in the visual cortex to render the image that we end up perceiving. The relationship between the colors that hit our eyes and the colors that we think we see is more complex than is conveyed above. For instance, context matters. If it is sunset and the entire landscape is illuminated with reddish light, our visual cortex can compensate and still report (albeit with lowered efficiency) a brown fox hiding under a green bush.

 

[ZapperZ]

We know that light of a specific colour has a specific frequency. Suppose we have a torch emitting white light, and we place a, say, red cellophane paper, in front of it. Now we would have red light. So does this mean the new beam we get has a new frequency? How?

Do not be confused with what your eyes observe with what it actually is. Your eyes, and your optical system, see the color "white" when a bunch of different frequencies are mixed together. This is not what an optical instrument observe, because such an instrument can distinguish different frequencies that make up white light.

It is why we do not perform spectroscopic experiments using our eyes as the detector.

Zz.

 

[Michelleeeee]

thanks a lot, everyone! :D