Spherical-Mirror Formula
With the Cartesian sign convention and paraxial rays, a spherical mirror satisfies 1/f = 1/v + 1/u.
Why this shows up in the exam
Mirror focusing · Image-position prediction · Concave-mirror laboratory measurements
Learn the idea
Object distance, image distance and focal length of a spherical mirror obey one reciprocal relation. Once two of object position, image position and focus are known, the third is fixed by the mirror geometry.
🧠 Memory hook: For mirrors, reciprocals of u and v add to 1/f.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- 1/f = 1/v + 1/u — paraxial spherical-mirror formula with signed distances
How to approach it
- 1Draw the principal axis and incident-light direction
- 2Assign signed u and f
- 3Solve for signed v and interpret it
Common slip-ups that cost marks
- •Substituting unsigned distances
- •Switching sign conventions midway
- •Using the lens formula sign pattern by memory
🌟 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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