Ray Optics and Optical Instruments

Physics · Class 12

Simulation · Physics · Class 12

Thin prism: deviation and colours

"Thin prism: deviation and colours" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "Refraction through a prism" in Ray Optics and Optical Instruments. A thin prism deviating red and violet by (n − 1)A, nearly independent of incidence, with the angular spread between colours. NEET has asked about this idea 2 times in 10 years of papers. JEE Main asked 66 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).

From the lesson Refraction through a prism in Ray Optics and Optical Instruments. Change the values and watch what happens.

Thin prism: deviation and coloursPhysics · Class 12

What it shows

NCERT § Refraction through a Prism

A thin prism deviating red and violet by (n − 1)A, nearly independent of incidence, with the angular spread between colours.

Controls

3 to try

  • Sliderprism angle 2–15°
  • Sliderangle of incidence 0–20°
  • Slidern_red / n_violet

Try this

Find the prism angle that deviates yellow light by 3° in n = 1.5 glass (6°), and show the deviation barely moves as incidence changes.

Common mistake

Using the thin-prism formula for a 60° prism.

In NEET

10 years of papers

Asked 2 times in 10 years of NEET.

See the past questions: Prisms, Dispersion and Optical Instruments, Ray Optics and Optical Instruments (chapter)

In JEE Main

42 shifts analysed

66 questions from Ray Optics and Optical Instruments across the 42 JEE Main shifts Lumi has analysed (2017–2026).

See the past questions

The idea behind it

NCERT § "Refraction through a Prism"

  • For a prism of refracting angle A, the angles of refraction at the two faces satisfy r₁ + r₂ = A, and the deviation is δ = i + e − A.
  • As the angle of incidence increases, the deviation first falls, reaches a minimum value D_m, and then rises again.
  • At minimum deviation the ray passes symmetrically through the prism: i = e and r₁ = r₂ = A/2; for the usual isosceles or equilateral prism the ray inside then runs parallel to the base.
  • Refractive index from minimum deviation: n₂₁ = sin[(A + D_m)/2] / sin(A/2).
  • For a thin prism (small A), D_m = (n₂₁ − 1)A, so the deviation does not depend on the angle of incidence for small angles.
  • Deviation depends on the refractive index of the prism relative to the surrounding medium; the same prism deviates light less when immersed in a liquid whose refractive index is lower than the prism's.

Questions

5 answers

What does the "Thin prism: deviation and colours" simulation show?
A thin prism deviating red and violet by (n − 1)A, nearly independent of incidence, with the angular spread between colours.
What mistake does the "Thin prism: deviation and colours" simulation fix?
A common mistake it clears up: using the thin-prism formula for a 60° prism.
Has NEET asked about this idea?
Yes. NEET has asked about this idea 2 times in 10 years of papers.
Does JEE Main ask questions from Ray Optics and Optical Instruments?
Yes. JEE Main asked 66 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
Is this simulation free?
Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Refraction through a prism" in Ray Optics and Optical Instruments (Class 12 Physics).

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