Total Internal Reflection and Paths in Prisms
At each prism face, determine the local incidence angle from geometry, then compare it with C where sin C = n_out/n_in; otherwise apply Snell's law.
Why this shows up in the exam
Binocular prisms · Periscopes · Retroreflecting prisms
Learn the idea
Prism ray paths often depend on whether a face transmits or totally reflects the internal ray. A prism can turn a beam like a mirror when the internal incidence at a face exceeds the critical angle.
🧠 Memory hook: At every face: geometry first, then compare with C.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- sin(C) = n_out/n_in — critical angle for an internal prism face
- delta = i + e - A — deviation when the ray exits through the second refracting face
How to approach it
- 1Trace the internal ray
- 2Find incidence at the next face
- 3Test TIR before applying an exit refraction
Common slip-ups that cost marks
- •Assuming every prism face refracts
- •Comparing the external angle with C
- •Using one normal for different faces
🌟 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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