Light – Reflection and Refraction — Part 4: Lenses, the lens formula and power: visual lesson, CBSE Class 10 Science

Lesson

Spherical lenses

Part 4 — how lenses form images, the lens formula, and the power of a lens that opticians write in dioptres.

In this lesson you will

Draw ray diagrams for convex and concave lenses for different object positions
Use the lens formula and magnification formula to solve problems
Calculate the power of a lens in dioptres and of lenses in contact

Concept

Convex and concave lenses

A convex lens is thicker in the middle; it converges parallel rays. A concave lens is thinner in the middle; it diverges parallel rays.

Table comparing convex and concave lenses by shape, effect on light, other name and example
	Convex lens	Concave lens
Shape	Thicker at the middle	Thicker at the edges
Effect on parallel rays	Converges to F₂	Diverges as if from F₁
Also called	Converging lens	Diverging lens
Example	Magnifying glass	Lens in spectacles for short sight

Concept

Lens terms and ray rules

The optical centre (O) is the centre of the lens; a ray through O passes undeviated. Each lens has two principal foci, F₁ and F₂, at the focal length f from O; 2F₁ and 2F₂ are at twice the focal length.

Table of three standard rays and how convex and concave lenses refract them
Incident ray	Convex lens: refracted ray	Concave lens: refracted ray
Parallel to the axis	Passes through F₂	Appears to come from F₁
Through F₁ (or towards F₂ for concave)	Emerges parallel to the axis	Emerges parallel to the axis
Through O	Goes straight on	Goes straight on

Cookies