# Color ( Frequency or Wavelength ) ?

1. Jul 16, 2006

### opticsman

Hi,

" what does the color we see depends on? Frequency or Wavelength ? "

2. Jul 16, 2006

### nazzard

Hello opticsman,

both are "connected" via the following equation:

$$\lambda=\frac{c}{f}$$

with $\lambda$: wavelength, $f$: frequency and $c$: speed of light (depending on the material it passes through).

To characterise the colour you can either use the wavelength or the frequency, one of them will suffice.

Regards,

nazzard

3. Jul 16, 2006

### Danger

Too right. Since the propogation velocity of the photons is invariable (within a particular medium), the frequency can be found from the wavelength and vise versa. Both terms are commonly used in place of 'colour' to describe light.

4. Jul 16, 2006

### Claude Bile

I think there is a subtlety here....

Do I see a different colour from the same vacuum wavelength, if I view it from air or in water? The answer to this is of course no, so it is tempting to say that the colour we see is frequency dependant, not wavelength dependant.

HOWEVER, ALL light we see must go through the liquid within our eyes (the vitrious humor), if this liquid has a constant refractive index (which one could probably safely assume it would), then we are back to square one - either answer is correct, because knowing one immediately defines the value of the other.

But what about other forms of light detection?? What will a CCD see if we stick them underwater (neglecting the disastrous effect on the electronics). Well, I'm fairly certain in this case, it is the frequency that matters, rather than the wavelength.

Claude.

5. Jul 17, 2006

### BoTemp

The frequency of a beam of light doesn't change from the creation to annihilation of the photon. This is not true of the wavelength, though.
$f \lambda = c/n$, where n is the index of refraction of the medium. The local n near the retinas will always be the same, so frequency and wavelength can be related if that's known.

If you want to generalize a bit and say "what specifies an electromagnetic plane wave" the answer would be the frequency (and direction of propagation). The wavelength is then determined by the formula above. This is all my way of saying the frequency is more an intrinsic property of a wave, wavelength more extrinsic.

Side note: The charts you see giving the color of light based on wavelength are based on wavelength in a vacuum, with n = 1.

In electronics, filters work with frequency, not wavelength. I don't know a great deal about how CCDs work, but I'm guessing E = hf is an fairly important formula. One would typically design a receiver to be sensitive to a particular range of frequencies, because once the photon enters the detector its' wavelength is determined by the local n; it doesn't matter what it was in the ocean.

For optical cavities, one designs such that the optical path length is an integer multiple of the wavelength. Basically, the two are highly related, although not completely interchangeable. The most convenient choice varies with application.

Last edited: Jul 17, 2006
6. Jul 17, 2006

### frdchang

in a doppler shift, would the energy content of a photon shift as well?

E=hF

perceived frequency seems to shift, so the percieved energy content seems like it will shift as well.

7. Jul 20, 2006

### opticsman

I ask my question in another way!

in a medium like water, for example, what would be happen for the red beam of he-ne laser ?
does it change its color in water ?

8. Jul 20, 2006

### Kurdt

Staff Emeritus
In going from one material to another the frequency of light is unchanged, however the speed and wavelength are changed. I do not know if there is any empirical evidence for the change of colours in a medium but somebody else may be able to clear that up. Anyway according to the model I presented if there is a change colour will be wavelength dependent and if not it will be frequency dependent.

9. Jul 20, 2006

### gato_

well, that's a good one. Actually, if you look through the net, many sources speak you about light frequency, but sensibility of the eye is spoken of in wavelength terms:
http://www.sjsu.edu/faculty/watkins/eye.htm
though i don't know the exact answer, my guess is for wavelength. Light has to interact with the cones in our eye to be detected, and the relative size between wavelength and receptor must be the parameter. My intuition is based in acoustics, where the relation between size of the sound producing organs and the sounds themselves is indeed with wavelength. So if it is so for production, it shoul for reception, isn't it?

10. Jul 20, 2006

### Claude Bile

My argument as to why I think frequency is important and not wavelength, is because photodetectors have detection sensitivities that ultimately depend on the energy band structure of the detector. (For example InGaAs detectors have cutoffs near 1000 nm (vacuum wavelength) this figure does not depend on the surrounding refractive index as we would expect if the wavelength was the critical parameter.

How this closely translates to the human eye I am not 100% sure. (Gato, the wavelength they refer to in the above link is the vacuum wavelength, and directly interchangable with frequency). Essentially the question boils down to whether it is photon energy or photon momentum (or the magnitude at least) that matters.

Claude.

11. Jul 22, 2006

### gato_

okay, you may be right. However , my common sense is that is size that matters for detection. Mightjust be wrong