Thin Lenses

Syllabus tag: KCSE | Physics | Form 4 | Topic 31 Thin Lenses

Lesson objectives

By the end of this topic, you should be able to:

• Describe the types of lens and define the terms used.
• Draw ray diagrams for converging and diverging lenses.
• Apply the lens formula and magnification formula.
• Describe optical instruments and defects of vision.

Summary

A converging (convex) lens is thicker at the centre and brings parallel rays to a real focus. A diverging (concave) lens is thinner at the centre and spreads rays from a virtual focus. The lens formula is 1/f = 1/u + 1/v, with f positive for converging lenses and negative for diverging ones.