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25B prisms n TIR

# 25B prisms n TIR - Mirages Physics 1C Lecture 25B Mirages...

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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). Huygens’s 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. Huygens’s Principle We can use Huygens’s 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 Let’s isolate congruent triangles ABC and ADC Huygens’s Principle ABC and ADC are congruent because they share hypotenuse AC and because AD = BC. We have: cos g = BC / AC and cos g ´ = AD / AC.

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