^Deviation of the emergent ray

^Deviation of the emergent ray

Incident ray deviates clockwise towards the base at first face by amount δ₁ (= i₁ – r₁) & then again  deviates clockwise towards the base at second face by amount δ₂ (= r₂ – i₂), thus the total deviation of the prism sum of δ₁ & δ₂ . Also from the cyclic quadilaternal AEHF,

r₁ + i₂ = A. From this information we can write

δ = δ₁ + δ₂ = i₁ + r₂ – A    [Called Prism equation

For given μ, λ & A with the increase of incidence angle deviation  angle first decreases &  becomes minimum & then starts increasing as shown in the graph.