Revise: Lenses
Refraction at a curved surface, the lens equation, image formation through lenses, and the power of a lens.
The Lens Equation
1/v − 1/u = 1/f relates object distance, image distance, and focal length.
An object at u=-30 cm with f=10 cm forms an image at v=15 cm.
Refraction Through Lenses
m = v/u; negative m means inverted image, |m|>1 means enlarged.
v=15 cm, u=-30 cm gives m=-0.5 (half-size, inverted).
Power of a Lens
P = 1/f (in dioptres, f in metres); shorter f means more powerful lens.
f = 0.25 m gives P = 4 D.
Refraction at a Curved Surface
The local normal varies across a curved surface, letting it focus or defocus light.
A water droplet focuses sunlight like a simple convex lens.