Refraction of light through a prism

Refraction of light through a prism :- Consider a monochromatic light ray PQ incident on the face AB of prism ABC of angle of prism A and refractive index μ.

Let

Angle of deviation(δ)

(Refraction of light through a prism)

Angle of deviation(δ): The angle through which the incident ray turns in passing through a prism is called angle of deviation. In the above figure if we extend the emergent ray RS backward then it meets the original direction of incident ray(PQ) at a point T. Here ∠KTR = angle of deviation(δ).

Graph between angle of deviation(δ) and angle of incidence(i)

(Refraction of light through a prism)

The graph of angle of deviation (δ) against the angle of incidence (i) is plotted and it is obtained as shown below :-

From the graph we observe that it is not symmetric about the minimum deviation position. Also if we draw a straight line parallel to i axis, it intersects the graph at two points. It means same deviation occurs at two angles of incidence i₁ and i₂. Here i₂ is called angle of emergence but from the principle of optical reversibility, the same angle i₂ may act as an angle of incidence, if the emergent ray falls on a plane mirror normally. In that case the ray will trace its path backward and emerge at an angle i₁. Then i₁ will acts as angle of emergence but the angle of deviation would remain the same.