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#light deviation

2 public questions tagged with this topic.

Refraction Through Prism and Minimum Deviation

In a prism, what condition results in the minimum deviation of light?

**Prism minimum deviation** δ_m satisfies n = sin[(A+δ_m)/2]/sin(A/2), A prism angle (degrees), n refractive index, δ_m minimum deviation. For A=60°, n=1.45, sin[(60+δ_m)/2]=1.45×sin30°=0.725, (60+δ_m)/2=46.5°, 60+δ_m=93°, δ_m=33°, illustrating n increase raises δ_m. In a prism, minimum deviation occurs when the angle of incidence equals the angle of emergence. This symmetry ensures the refracted ray inside the prism is parallel to the base, minimizing the deviation angle. Substituting values gives Angle of incidence equals angle of emergence, which matches expected image position and magnification from mirror/lens formula 1/f = 1/v - 1/u (lens) or 1/f = 1/v + 1/u (mirror), confirming sign conventions and refraction principles.

Ref: NCERT > Physics Book > Ray Optics > Refraction Through Prism and Minimum Deviation

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Refraction at Plane Surfaces and Snell's Law

In a prism, what causes the emergent ray to deviate from the incident ray’s path?

**Refractive index** n = c/v, water 1.33 means light 1.33 times slower than vacuum. Passing from water to air at 49°, n₁ sinθ₁ =1.33×sin49°≈1.33×0.755=1.004>1, so sinθ₂>1 impossible, total internal reflection occurs, no refraction. The deviation in a prism occurs because light refracts at both the entry and exit faces, bending toward the normal when entering (denser medium) and away from the normal when exiting (rarer medium). The prism’s geometry and refractive index determine the total deviation angle. Substituting values gives Refraction at the prism’s interfaces, which matches expected image position and magnification from mirror/lens formula 1/f = 1/v - 1/u (lens) or 1/f = 1/v

Ref: NCERT > Physics Book > Ray Optics > Refraction at Plane Surfaces and Snell's Law

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