# Knowing the frequency of light

#### jalalmalo

I was watching a movie talking about the hydrogen atom and the study of spectrum from hydrogen gas when a question poped into my head. How can we know the frequency of light from observing its colour? Please give example of experiments done to confirm that.

Thank you all in advance

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#### f95toli

Gold Member
The easiest way is to simply use a grating to measure the wavelength and then calculate the frequency using f=c/lambda.

This experiment will obviously not measure the colour directly, simply because colour is just how we percieve light (and then only in the visible spectrum); it is not "physical".

Hence, the only way to relate frequency and colour is to first measure the frequency and then ask someone what colour the light is. There are plenty of tables where you can find this information.

#### Cantab Morgan

Great points, f95toli! When I first read the post, I was confused because I thought, "well, the definition of color is its frequency." But of course, color is a very anthropocentric thing.

It's interesting to point out that we don't all have identical biology, and so we don't all perceive colors the same way. Most of us have three different kinds of detectors to disambiguate colors, but some folks have only two or even one. A handful of lucky women on the planet actually have four, and I wonder what it must look like to see through their eyes.

#### alxm

Not to mention: Many colors, some of which we humans perceive as distinct colors in their own right, like white and magenta, don't correspond to any single frequency.

The human range of perceptible colors is usually pictured on a chromaticity diagram that looks like http://www.ladyada.net/learn/proj1/542px-CIExy1931_CIERGB.png" [Broken] (which is only an approximation since your computer monitor only reproduces a smaller area inside the diagram).

The single-wavelength (monochromatic) colors of the visible spectrum follow the outer curved edge, with the inside being mixtures of those wavelengths. In the middle near the bottom you find white, a mix of red, green and blue. (Or cyan-magenta-yellow!)

You can also see that there are a lot more 'greens' than any other color - we're most sensitive to green light. (The diagram is, roughly 'perceptually uniform', meaning that the same distance anywhere the diagram corresponds the the same percieved amount of difference in color. It's not quite apparent though since, again, your monitor doesn't actually reproduce the entire thing).

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#### Cantab Morgan

You can also see that there are a lot more 'greens' than any other color - we're most sensitive to green light. (The diagram is, roughly 'perceptually uniform', meaning that the same distance anywhere the diagram corresponds the the same percieved amount of difference in color. It's not quite apparent though since, again, your monitor doesn't actually reproduce the entire thing).
That's fascinating, I didn't know that. I wonder if we're so good at seeing greens because so much of what we can eat and not eat is green. Those ancestors who were good at telling the difference, and identifying ripe stuff might have had an advantage.

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