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Unformatted text preview: 1 Physics 1C Lecture 25B "If you want the rainbow, you gotta put up with the rain."--Dolly Parton Mirages Mirages occur because of total internal reflection (TIR). Hot air is less dense than cool air. The less dense hot air has a small index of refraction. The more dense cool air has a higher index of refraction. TIR happens for light in cool air (slow) off of hot air (fast). Huygenss Principle Christiaan Huygens (1629-1695) introduced a geometric construction that helped to understand many practical aspects of propagation of light. In this construction, all points of a wave front are taken as point sources for the production of secondary waves, called wavelets . New position of the wave front after time D t has elapsed is the surface tangent to the wavelets. Huygenss Principle We can use Huygenss principle to derive the laws of reflection and refraction. AB is a wave front of incident light. Point A sends out a wavelet toward point D. Point B sends out a wavelet toward point C. Both rays move with the same speed: AD = BC = c D t Lets isolate congruent triangles ABC and ADC Huygenss Principle ABC and ADC are congruent because they share hypotenuse AC and because AD = BC....
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This note was uploaded on 02/11/2012 for the course PHYS 1C 1C taught by Professor Wethien during the Spring '11 term at UCSD.
- Spring '11