# Why wavelength of light changes while its frequancy remains the same in refraction?

1. Feb 21, 2012

### maizaz

When light or any other electromagnetic wave enters from one medium to another, there is a change in its speed. Speed of a wave is given as v = frequency x wavelength. My question is, what does change, frequency, wavelength or both?

If you say..... this change in speed is generally attributed to change in wavelength. then again my question is, why the medium discriminates between wavelength and frequency of the incoming wave?

2. Feb 21, 2012

### bp_psy

Re: Why wavelength of light changes while its frequancy remains the same in refractio

The medium only determines the speed of the wave it does not destroy any portion of the wave nor does it generate anything.The information is conserved.Imagine you have two connected wires with different densities. If you send mechanical pulse you will see that the speed and length change as it passes from one wire to the other but the pulse will not disappear or be split into multiple pulses.

3. Feb 22, 2012

### maizaz

Re: Why wavelength of light changes while its frequancy remains the same in refractio

Well and good bp_psy, but i want to know why we always say that wavelength changes and frequancy remains the same as the speed of the wave (it will be better consider light waves) took a different value upon refraction.

4. Feb 24, 2012

### Claude Bile

Re: Why wavelength of light changes while its frequancy remains the same in refractio

Nature requires that the phase of an EM wave must remain continuous across a boundary. In order for the phase to be continuous for all time the frequency must be constant.

Therefore, it is the wavelength that must change upon a change in velocity.

Claude.

5. Feb 24, 2012

### sophiecentaur

Re: Why wavelength of light changes while its frequancy remains the same in refractio

It's the other way round. The difference in Propagation Speed makes the wavelength different. The speed is the primary quantity - a single pulse (with no explicit wavelength - no repetition) will still propagate at a speed through a medium.

6. Feb 24, 2012

### maizaz

Re: Why wavelength of light changes while its frequancy remains the same in refractio

The answer given by Claude Bile seems more reasonable to me, thank you all.

7. Feb 27, 2012

### luckis11

Re: Why wavelength of light changes while its frequancy remains the same in refractio

From Thomas Young's experiments it was concluded that:
(the distances between the interference tossils on the film are that much)=>
(the distance between two most macrocosmic wavefronts at the spece between the slits and the film is the waveleghth λ)
thus that the distance between two wavefronts of the red light λ1, and it's λ2 of the blue light. But (if the definition of frequency is how many wavefronts pass through a surface in 1 sec), the conclusion that the frequency of red light is f1 and of blue light is f2, needs both the previous conclusion, and the supposition that the wavefronts of both the red and the blue light run with the same speed c, otherwise the frequencies of red and blue are not the known f1 and f2.

Now, can you explain what you mean by "the phase of an EM wave must remain continuous across a boundary. In order for the phase to be continuous for all time the frequency must be constant. Therefore, it is the wavelength that must change upon a change in velocity"? The wavelength could change if the wavelength is not the only difference between red and blue light. But that "the wavelength must change in order for the frequency to remain the same because..."?

Last edited: Feb 27, 2012