The equation ω=μv-μr was derived for a prism having small refracting angle. Is it also valid for a prism of large refracting angle? Is it also valid for a glass slab or a glass sphere?

Dispersive power depends on angular deviation,


ẟ= (μ-1) A


Where, A = refracting angle


Above equation is valid only for small value of A and angle of incidence. Therefore, dispersive power is not valid for prism with large refracting angle.


a. For a glass sphere, the refracting angle will be large. Hence it is not valid for a glass sphere.


b. For a glass slab, the refracting angle A=0. But, ẟ = (μ-1) A = 0. So this equation for ⍵ is not valid in this case.


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