Plane-Mirror Image and Minimum Mirror Size
For a stationary plane mirror, image distance equals object distance in magnitude and linear magnification is +1; a person needs a mirror of half their height to see the full body.
Why this shows up in the exam
Dressing mirrors · Rear-view alignment · Optical lever setups
Learn the idea
A plane mirror forms a virtual, erect, same-size image as far behind the mirror as the object is in front. The image looks like a symmetric copy behind the mirror even though no light actually meets there.
🧠 Memory hook: Plane mirror: equal distance, equal size, virtual side.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- v = -u — Cartesian-sign relation for a plane mirror
- m = +1 — erect same-size virtual image
How to approach it
- 1Reflect the object point across the mirror
- 2Join image points to the eye if visibility is asked
- 3Use similar triangles for mirror length
Common slip-ups that cost marks
- •Adding observer distance to minimum mirror length
- •Calling the image real
- •Using full body height as minimum mirror height
🌟 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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