Minimum Deviation and Prism Refractive Index
For a prism in air at minimum deviation, i=e, r1=r2=A/2 and n = sin[(A+delta_m)/2]/sin(A/2).
Why this shows up in the exam
Prism spectrometers · Material index measurement · Wavelength calibration
Learn the idea
At minimum deviation the prism path is symmetric, allowing refractive index to be found directly. The least-deviated ray enters and leaves at equal angles, so the internal ray splits the prism angle equally.
🧠 Memory hook: Minimum deviation means a symmetric path.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- n = sin((A+delta_m)/2)/sin(A/2) — prism index in air at minimum deviation
- i = e; r1 = r2 = A/2 — symmetric minimum-deviation path
How to approach it
- 1Confirm minimum-deviation wording
- 2Set the symmetric angles
- 3Apply the sine-ratio formula
Common slip-ups that cost marks
- •Setting i=A/2 instead of r=A/2
- •Using ordinary deviation in the minimum formula
- •Ignoring the external medium
🌟 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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