Prism — Part 1: Refraction through a prism and minimum deviation: visual lesson, ISC Class 12 Physics

Lesson

Refraction through a Prism

Part 1 — how a prism bends light, the angle of deviation, and the special symmetric path of minimum deviation.

In this lesson you will

Trace a ray through a prism and prove that r1 + r2 = A and δ = i + e − A
Interpret the graph of deviation against angle of incidence, including the limits of emergence
Derive n = sin((A + δm)/2)/sin(A/2) and use it to find refractive indices

Concept

The parts of a prism

A prism is a transparent block with two plane refracting faces inclined to each other.

Refracting edge: the line where the two faces meet. Refracting angle A (angle of the prism): the angle between the faces.
Base: the face opposite the refracting edge.
Principal section: a cut at right angles to the edge; we draw rays in this plane.
For a prism denser than its surroundings, a ray is bent towards the base at both faces.

Diagram

Tracing a ray through a prism

A ray meets the first face at angle of incidence $i$ , refracts at $r_1$ , crosses the prism, meets the second face at $r_2$ and leaves at the angle of emergence $e$ . Snell’s law at each face: $\sin i = n\sin r_1 \qquad n\sin r_2 = \sin e$

All four angles are measured from the normals to the faces, not from the faces.
The angle of deviation δ is the angle between the incident ray produced forward and the emergent ray.

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