Applet by Fu-Kwun Hwang
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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:
Suspend/Resume the animation:
*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.