Lens Maker's Formula
For a thin lens in air, 1/f = (n-1)(1/R1-1/R2). In a medium replace n by the relative index n_lens/n_medium.
Why this shows up in the exam
Spectacle-lens manufacture · Camera-lens design · Index measurement
Learn the idea
A lens's focal length is set by its surface curvatures and refractive index relative to its medium. Stronger curvature and a larger index contrast bend rays more and therefore increase optical power.
🧠 Memory hook: Lens power equals index contrast times curvature difference.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- 1/f = (n-1)(1/R1-1/R2) — thin lens in air
- 1/f_m = (n_l/n_m-1)(1/R1-1/R2) — thin lens immersed in medium n_m
How to approach it
- 1Mark light direction and curvature centres
- 2Assign signed R1 and R2
- 3Use relative index for the surrounding medium
Common slip-ups that cost marks
- •Using unsigned radii
- •Using absolute lens index after immersion
- •Applying the formula to a thick lens without qualification
🌟 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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