# The color of light can be change by the change in wavelength?

1. Feb 2, 2008

### pawan_ctn

the color of light can be change by the change in wavelength?

Each color of light has specific range of wave length.So if light will go in dense medium then its velocity will decrease...Acc to equ..
Velo=freq *wavelength

so wave length also will decrease bcoz frequency is constant...

2. Feb 2, 2008

### _Mayday_

The velocity will decrease but will be constant within that medium.

$$v=\lambda f$$

$$v=velocity$$
$$\lambda=wavelength$$
$$f=frequency$$

Here, through a given medium "v" is constant. An increase in wavelength will result in a decrease in frequency and a decrease in wavelength will result in an increase in frequency.

3. Feb 2, 2008

### pawan_ctn

means freq can be change?

4. Feb 2, 2008

5. Feb 2, 2008

### _Mayday_

If "v" decreases and is still moving at a constant velocity, then there will be a change in wavelength and thus a change in frequency. Is there a homework question related to this, or is it just out of interest?

6. Feb 2, 2008

### pawan_ctn

but freq doesnt change.........

7. Feb 2, 2008

### _Mayday_

Erm...frequency can change, if a police car is driving towards you with it's siren on, you will hear a higher pitch (frequency) than if it is moving away in which case the pitch (frequency) will be lower.

8. Feb 2, 2008

### _Mayday_

I have some information off wikipedia on this subject.

In passing through materials, the observed speed of light can differ from c. The ratio of c to the phase velocity of light in the material is called the refractive index. This apparent contradiction to the universality of the constant c is a consequence of sloppy (but universally practiced) nomenclature: what is referred to as light in a medium is really a light-like hybrid of electromagnetic waves and mechanical oscillations of charged or magnetic particles such as electrons or ions, whereas light in the strict sense is a pure electromagnetic wave (see further discussion below). The speed of light in air is only slightly less than c. Denser media, such as water and glass, can slow light much more, to fractions such as ¾ and ⅔ of c. Through diamond, light is much slower—only about 124,000 kilometres per second, less than ½ of c.[5] This reduction in speed is also responsible for bending of light at an interface between two materials with different indices, a phenomenon known as refraction.

Since the speed of light in a material depends on the refractive index, and the refractive index depends on the frequency of the light, light at different frequencies travels at different speeds through the same material. This can cause distortion of electromagnetic waves that consist of multiple frequencies, an effect called dispersion.

Note that the speed of light referred to is the observed or measured speed in some medium and not the true speed of light (as observed in vacuum). It may be noted, that once the light has emerged from the medium it changes back to its original speed and this is without gaining any energy. This can mean only one thing—that the light's speed itself was never altered in the first place.

It is sometimes claimed that light is slowed on its passage through a block of media by being absorbed and re-emitted by the atoms, only traveling at full speed through the vacuum between atoms. This explanation is incorrect and runs into problems if you try to use it to explain the details of refraction beyond the simple slowing of the signal.

Classically, considering electromagnetic radiation to be like a wave, the charges of each atom (primarily the electrons) interfere with the electric and magnetic fields of the radiation, slowing its progress.

The full quantum-mechanical explanation is essentially the same, but has to cope with the discrete particle nature (see Photons in matter): the E-field creates phonons in the media, and the photons mix with the phonons. The resulting mixture, called a polariton, travels with a speed different from light.

http://en.wikipedia.org/wiki/Speed_of_light

9. Feb 3, 2008

### Claude Bile

Boundary conditions demand that the electromagnetic field be continuous. This is only possible if the electromagnetic field on either side of the interface between the two media is equal. Hence frequency is invariant.

Any change in velocity is accompanied by a change in wavelength, but not frequency.

Claude.

10. Feb 4, 2008

### pawan_ctn

but my question is ,can the color of light be change by the change in wavelength?

11. Feb 4, 2008

### ZapperZ

Staff Emeritus
I think people are missing pawan_ctn question here. pawan_ctn: this is an example where you should be very explicit in clearly defining your question, especially when you're asking for something very subtle.

I believe if you look at his first post, he's asking for the case where light goes from one medium to another. As claude has pointed out, the frequency of light does not change, but the speed of light and, consequently, the wavelength, do. Now, the question is, if I live in one medium, while another person lives in the other medium, will we both "see" a different color, simply due to the change in wavelength, but NOT a change in frequency?

Did I get this right?

Zz.

12. Feb 4, 2008

### Claude Bile

If indeed this is the OPs question...

Colours are differentiated on the basis of photon energy (or, equivalently frequency) and not wavelength, both in biological detection systems (eyes) and in just about any standard light detection scheme I can think of.

You would therefore see the same colours underwater as you would in the open air.

Claude.

Last edited: Feb 4, 2008
13. Feb 4, 2008

### Loren Booda

If the medium (e. g., water) were dense or extended enough, would perceived colors then be frequency shifted? Would frequency shift then be symmetric for both observers?

14. Feb 5, 2008

### ZapperZ

Staff Emeritus
Did you somehow missed the part that says "... the frequency of light does not change... "?

Zz.

15. Feb 5, 2008

### Staff: Mentor

And even if color wasn't determined by frequency, when the light reaches your retina it would have the same wavelength anyway, even under water.

16. Feb 5, 2008

### Loren Booda

I was thinking that even gravitational redshift would be symmetric (overall the same) between two observers.