Multiple Plane Mirrors and Repeated Images
For two plane mirrors inclined at angle theta, the standard image-count rule depends on 360 degrees/theta and on object symmetry; direct construction is safest for exceptional cases.
Why this shows up in the exam
Kaleidoscopes · Mirror mazes · Multipass optical paths
Learn the idea
Repeated reflection between plane mirrors creates a countable sequence of virtual images. Each mirror can reflect the object and images made by the other mirror, producing a geometric pattern.
🧠 Memory hook: Count sectors first; then check symmetry and overlap.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- n = 360 degrees/theta — auxiliary ratio used in the image-count rule
- N = n - 1 — when n is an integer and the symmetric standard case applies
How to approach it
- 1Sketch successive reflected object positions
- 2Evaluate 360 degrees/theta
- 3Check integer parity and object placement
Common slip-ups that cost marks
- •Applying N = 360/theta - 1 without checking conditions
- •Counting coincident images twice
- •Mixing physical reflections with virtual image count
🌟 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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