Thin-Prism Approximation
For a thin prism in air under the paraxial small-angle approximation, delta = (n-1)A, with A and delta in the same angular unit.
Why this shows up in the exam
Small beam deflectors · Prism combinations · First-order dispersion estimates
Learn the idea
A small-angle prism deviates light by approximately its angle times the refractive-index excess. For a weak prism, all ray angles are small enough that sine and angle are nearly the same in radians.
🧠 Memory hook: Thin prism: deviation is (n-1) times A.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- delta = (n-1)A — thin prism in air at small angles
How to approach it
- 1Check that the prism and ray angles are small
- 2Use relative index if not in air
- 3Apply delta=(n_rel-1)A
Common slip-ups that cost marks
- •Using the formula for a large prism
- •Mixing degrees and radians in derived calculations
- •Omitting the surrounding-medium correction
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A real object is placed 30 cm from a converging lens of focal length 10 cm. Find the real image distance.
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