The world of light, read from a single idea: superposition

A traveling wave moves at v = fλ, and when two waves meet, crests add while a crest and a trough cancel. Slightly different frequencies make beats.

Waves from two coherent point sources reinforce when their path difference is a whole number of wavelengths and cancel at half-integers, laying down a regular pattern of bright and dark.

The double slit brightens at d sinθ = mλ, spacing the fringes by Δy = λL/d on the screen. Here we watch it as a continuous light intensity; the single-quantum view lives in the quantum-mechanics area.

A single slit makes a sinc² intensity that darkens at a sinθ = mλ, while a grating stacks many slits into sharp maxima. Understand it as a sum of Huygens wavelets.

Light bends at a boundary by n₁ sinθ₁ = n₂ sinθ₂ and, past the critical angle, reflects totally. A lens forms real and virtual images through 1/f = 1/u + 1/v.

Light vibrates transversely, so a polarizer passes only one direction. The transmitted intensity follows Malus law I = I₀cos²θ, and crossed filters extinguish the light.