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Unformatted text preview: Physics 1C Lecture 25B 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). Fata Morgana Huygenss Principle Christiaan Huygens (16291695) 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 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 t Lets isolate congruent triangles ABC and Huygenss Principle ABC and ADC are congruent because they share hypotenuse AC and because AD = BC. We have: cos = BC / AC and cos = AD / AC. Therefore, cos = cos and = This gives 1 = 1 which is the law of reflection Prisms Example An incident ray in air is headed straight...
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This note was uploaded on 01/30/2012 for the course PHYS 1C 1C taught by Professor Wethien during the Spring '11 term at UCSD.
 Spring '11
 Wethien

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