Thin-Lens Formula
For a paraxial thin lens under the Cartesian convention, 1/f = 1/v - 1/u.
Why this shows up in the exam
Camera focusing · Projector placement · Image-distance calculations
Learn the idea
A thin lens links object distance, image distance and focal length through signed reciprocals. A converging lens spends its focusing ability partly cancelling incoming divergence and partly creating outgoing convergence.
🧠 Memory hook: Lens formula: 1/f = 1/v - 1/u with signed distances.
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 — signed thin-lens formula in a uniform surrounding medium
How to approach it
- 1Draw the lens and positive axis
- 2Assign signed u and f
- 3Solve for v and classify the image
Common slip-ups that cost marks
- •Using the mirror formula
- •Substituting all positive distances
- •Failing to interpret a negative image distance
🌟 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.
More from Optics
Interference of light
Examine the principle of superposition, Young's double slit experiment, fringe width, intensity distribution, and the conditions for constructive and destructive interference.
Diffraction of light
Understand the bending of light around obstacles, single slit diffraction patterns, their width, and the effect of wavelength on diffraction.
Lenses and mirrors
Explore the image formation, ray diagrams, lens and mirror formulas, and the behavior of light with concave/convex lenses and mirrors, including combinations and virtual objects.
Optical instruments
Understand the working principles, magnification, resolving power, and design of devices like microscopes and telescopes, including their adjustments and measurement techniques.
Polarization of light
Learn about the polarization of light, Brewster's law, Malus' law, and the use and function of polaroids.
Dispersion and rainbow formation
Study how light splits into its constituent colors through dispersion in prisms and natural phenomena like rainbows, including minimum deviation and dispersive power.