Reflection and refraction

Applet by Fu-Kwun Hwang

When a beam of light impinges at some angle on the smooth flat surface of an optically dense medium, the wave "sees" a vast array of very closely spaced atoms that will somehow scatter it. (At the wavelengths of light -- d=500nm -- the Earth's atmosphere at STP has about 106 molecules in such a d3-cube). As the wavefront descends, it excites one scatterer after another, each of which reradiates a stream of photons that can be thought of as a hemispherical wavelet in the incident medium. Because the wavelength is so much greater than the separation between the molecules, the wavelets advance together and add constructively in only one directions, and there is one well-defined reflected beam. The wavelets bend as they cross the boundary, because of the speed change. For similar reason, they form one well-defined refracted beam.

Instructions on how to use the animation below:

  1. The whole area has been divided into two regions (Top/Buttom).
  2. Click within the black semicircle near the top, then drag the mouse (Left-Right) to change the angle of incidence.
  3. You can enter the ratio of the index of refraction for those two media (n2/n1).
  4. Click Faster/Slower to change the wave speed.

Suspend/Resume the animation:

  1. The animation is suspended when you press down the left mouse button and resumed when you release the mouse button.
  2. If you press the right mouse button, the animation is also suspended. But you will have to press the right mouse button again to resume the animation.
  3. What if I press the right mouse button first then press the left mouse button? Try it!

*For sound waves originating in and impinging on water:
If the incident angle is larger than critical angle (about 13o), the sound wave will be totally reflected. No sound will be transmitted.