# B Refraction of a wave

1. Oct 11, 2018

### jha192001

(1)What is the real cause of refraction?
Light wave cannot possible have a change of speed in going onto diffrent medium hence what is going on inside actually that we say its a change of speed.
(2) I am notable to comprehend how a change in wavelength not always means a change in frequency. If i draw a diagram of a way of a axis a change in wavelength always results in change in frequency.
Thanks

2. Oct 11, 2018

### jbriggs444

Why do you claim this?

3. Oct 11, 2018

### ZapperZ

Staff Emeritus
This is puzzling, because assuming that you are just learning about it, why would you make such an erroneous statement?

Please note that, if you want to be technical about it, the speed that we measure when light goes from one medium to another, is the group velocity of the wave. This definitely changes as exhibited by the different index of refraction.

Otherwise, you have a lot of explaining to do.

Zz.

4. Oct 11, 2018

### Drakkith

Staff Emeritus
The change in wavelength occurs because the speed of the wave changes. The wavelength and frequency of a wave is given by the equation $v=fλ$, where v is the speed of the wave, f is the frequency, and λ is the wavelength.

You can think of the frequency of an EM wave as being set and the wavelength changing as the speed changes. So a wave that enters a medium with a higher refractive index slows down, forcing the wavelength to change in order to keep the frequency the same. As an extreme example, if the speed of the wave is cut in half, then the wavelength is also cut in half. Otherwise the equation would be wrong.

5. Oct 11, 2018

### jha192001

But still its not clear how does refraction occur.
A change in speed doesn't signify change in direction. What cause the light to change the direction the same everytime

6. Oct 11, 2018

### jha192001

I stated the above statement cause i remembered a video in which they told that speed of light doesnt change in a medium infact light is absorbed and desorped by atoms and hence it appears it has slowed down tho it has it just follows a longer path. I watched the video a time ago and while i was learning about refraction i came across this doubt i had

7. Oct 11, 2018

### ZapperZ

Staff Emeritus
If it is not clear, then you shouldn't be making such silly statement in the first place. You should simply ask why there is a direction change. Question: What do you think will happen to the wavefronts when they cross the interface?

Note: there is NO direction change if the angle of incidence is perpendicular to the interface. Yet, there will still be a change in the group velocity.

This is not sufficient. You should make explicit reference to the video. After all, you could easily have misinterpreted what the video is saying. The speed of PHOTONS may not change inside the medium. But as I've stated before, this is NOT the speed of light that is being measured! You seem to not know the concept of group velocity in a medium.

Zz.

8. Oct 11, 2018

### Drakkith

Staff Emeritus
Think about Huygen's principle. As the wave hits the medium boundary, the source wavelets at that boundary interfere with each other to produce the new wavefront. As illustrated below, the slower speed of the wavelets in the new medium is what gives rise to the change in direction.

That's not quite correct. Near-field effects inside the medium affect the EM wave, and this can't really be explained very well as photons being absorbed and re-emitted like you normally imagine. Besides, a classical EM wave is simply the sum effect of huge numbers of photons, so we're talking about the collective effect of them all. And if they are being absorbed and re-emitted individually, then as a whole collective you will have large numbers of photons in each possible state (emitted and traveling, absorbed, etc) and the net effect is to slow down the EM wave as a whole. But please don't take my explanation of this as anything close to accurate. You would need to get into quantum electrodynamics to understand just what is going on.

9. Oct 11, 2018

### jha192001

(1) Everybody ask a question thinking what they ready know to be correct. Thats what i did. Maybe you failed to understand this simple thing. I dont have a reference to video thats why i didnt stated it first and stated when needed. The only new thing you said were about the group velocity and i will read about them, its a.new term to me(or you can send a link if you are kind enough).
And nobody is understanding the basic ques. of what really causes Refraction.Light isnt bound to follow our rules right, so why change in medium=change in direcn. upon change in speed.
Maybe the answer is beyond my present scope to understand but when i ask questions, wise people tend to help in providing links or terms that may increase my knowledge.
Thanks

Last edited by a moderator: Oct 11, 2018
10. Oct 11, 2018

### jha192001

Thanks this is best far..Thanks all of you

11. Oct 11, 2018

### Staff: Mentor

That's not a good strategy at the PF. Much better is to post links to the reading you have been doing so far, and ask specific questions about what is confusing you in that reading.

12. Oct 11, 2018

### Staff: Mentor

The refraction of light at a dielectric boundary is similar in concept to the refraction of water waves as they pass from an area of one water depth to another. You can see this near the beach where the depth of the water is changing, and the waves may be coming in at an angle to the beach. Have you seen figures like the one below before?

http://www.geosci.usyd.edu.au/users...GIS/Tsunami/Images/Refraction-Water-Waves.gif

13. Oct 11, 2018

### jbriggs444

Wise men, upon encountering an apparent contradiction, begin by questioning what they think they know.

“The most difficult subjects can be explained to the most slow-witted man if he has not formed any idea of them already; but the simplest thing cannot be made clear to the most intelligent man if he is firmly persuaded that he knows already, without a shadow of a doubt, what is laid before him.”

14. Oct 11, 2018

### jha192001

yes sir, was i stubborn when any of my knowledge was corrected? Thanks for the quote.

15. Oct 11, 2018

### sophiecentaur

I can see what your problem is / was. You started off saying that the speed of light doesn't change in a medium. I think you have now accepted that this is not the case and that c=λf must be applied correctly and relevantly (your first post). The thing that does not (cannot) change is the frequency. So the wavelength must be the thing that changes when c changes. At any boundary, there has to be continuity of phase - which is to say that the peaks of all the waves as they traverse the boundary cannot suddenly change position. That means that the angle of the wave must change (see the diagrams above) in order to support incident and refracted waves having different wavelengths.

16. Oct 13, 2018

### Lord Jestocost

As Richard Feynman states it in "The Feynman Lectures on Physics, Volume I": "Before we proceed with our study of how the index of refraction comes about, we should understand that all that is required to understand refraction is to understand why the apparent wave velocity is different in different materials." Have a look at chapter 31 "The Origin of the Refractive Index" in http://www.feynmanlectures.caltech.edu/I_toc.html

17. Oct 13, 2018

### DrStupid

Even single photons behave like light waves inside the medium, including reduced speed and refraction.