Refraction
The bending of a light ray as it passes obliquely from one transparent medium into another with a different refractive index.
Governing formula
n1 sin θ1 = n2 sin θ2 (Snell's Law)
SI unit
Dimensionless (n)
In depth
Refraction occurs due to the change in light propagation speed between media ($v = c/n$). Light bends toward the normal when entering a denser medium ($n_2 > n_1$) and away from the normal when entering a rarer medium.
Examples in the real world
A straight drinking straw appearing bent or broken when placed partially in a glass of water.