Interference and Evanescent Waves

Most recent answer: 12/31/2015

Q:
Can you please tell me how destructive waves gives rise to evanescent waves in a prism? what is the physics of interference for the evanescent waves to occur on the surface of a prism by incoming laser light?
- Ashutosh Mukherjee (age 25)
Chemnitz
A:

It's interesting to think about evanescent waves as an interference effect. Think of what happens as some of the wave reflects at the glass surface. That surface has a pattern of oscillating electric fields on it. (Magnetic too, but we needn't consider them now.) You can think of this pattern as alternating stripes of field oscillating up and down, acting like antennas sending a wave out into the air.  Thus the frequency of the wave going out into the air will matches the frequency of the wave inside the glass. Since the wave travels faster in the air than in the glass, it's wavelength will be longer. 

If the wave in the glass is nearly parallel to the surface, those alternating stripes are spaced just a little farther apart than the wavelength inside the glass. Then for any direction outside the glass, those radiating antennas are spaced so closely together  that the wave at a distance will come from a mixture of opposite stripes, and will fall to zero quickly as it goes farther away. In other words, there's no direction outside the glass for which the wave from one stripe is just shifted by one wavelength from the wave from the next stripe. The combination of the waves coming from the different stripes causes a mixture of all possible phases (once you've gone back away from the surface), giving destructive interference.

This is all much easier to see with a picture, but unfortunately our image upload isn't working right now. You could have a look at this one: .

Mike W.


(published on 12/31/2015)