Refraction — light that bends

Cross a boundary at an angle and light changes speed — so it changes direction. One line, n₁ sin θ₁ = n₂ sin θ₂, runs through everything here: the bent straw, the shallow-looking pool, the sparkle of a diamond, the internet in a glass thread. Grab a ray and drag it; the geometry keeps Snell's law true.

Snell's law, by hand

Set the index on both sides and drag the incident ray. Into a denser medium (n₂ > n₁) the ray bends toward the normal; the other way it bends away and the pencil diverges — push it far enough and nothing gets through at all. Across the boundary the product n·sin θ always matches.