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W11Physics1CLec25Bfkw

# W11Physics1CLec25Bfkw - Physics 1C Lecture 25B Mirages...

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Physics 1C Lecture 25B

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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

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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 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 t Let’s isolate congruent triangles ABC and ADC

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Huygens’s Principle ABC and ADC are congruent because they share hypotenuse AC and because AD = BC. We have: cos γ = BC / AC and cos γ ´ = AD / AC. Therefore, cos γ = cos γ ’ and γ = γ This gives θ 1 = θ 1 ’ which is the law of reflection
Prisms Example

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W11Physics1CLec25Bfkw - Physics 1C Lecture 25B Mirages...

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