Unlike a glass slab, a prism's slanted faces bend light sideways — that sideways bend is the angle of deviation.
PE = incident ray, EF = refracted ray, FS = emergent ray. D = angle of deviation.
You already know that in a rectangular glass slab, the two refracting surfaces are parallel — so the emergent ray comes out parallel to the incident ray, just shifted sideways a little.
A prism is different: it has two triangular bases and three rectangular faces that are inclined to each other, not parallel. Light enters through one slanted face and exits through another.
As light enters the glass at the first surface, it bends towards the normal (going from a rarer to a denser medium). At the second surface, going from glass back to air, it bends away from the normal. Because the two surfaces aren't parallel, these two bends don't cancel out — the ray exits at a real angle compared to how it entered.
That total change in direction between the original incident ray and the final emergent ray is called the angle of deviation. It's the prism's peculiar wedge shape that makes this deviation possible — a slab can't do this.
Key exam points
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Refraction of Light Through a Triangular Prism | Angle of Deviation | CBSE Physics · CBSE Physics