Sunday, 21 October 2018

optics - Analytic solution for angle of minimum deviation?



enter image description here


Consider a simple prism with a prism angle A, angle of incidence θ1, angle of emergence θ4 and the first and second angle of refraction as θ2,θ3. the refractive index for the prism (w.r.t the surroundings) is n. The angle of deviation is δ.I wanted to derive an equation that could give the relation between θ1 and δ, plot of which for a monochromatic light is as in the animation here. Below is my failed attempt (equations 2 and 3 are from the geometry of the figure):- θ4=sin1nsin(θ3)

A+δ=θ1+θ4
A=θ2+θ3
δ=θ1+sin1nsin(θ3)A
δ=θ1+sin1nsin(Aθ2)A
δ=θ1+sin1nsin(Asin1sin(θ1)n)A
Equation, when I plotted it on Wolframalpha for an equilateral prism with n=1.5 yielded the required plot in the limit 28.5<θ1<90 (to avoid total internal reflection). But then, how do I use this equation, to analytically find the angle of minimum deviation, and the fact that at minimum deviation θ1=θ4. (I tried taking the derivative, but it turned out to be too complex).




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