Refraction and Snell's Law

Why light bends when it changes medium.

Light changes speed when it passes into another transparent medium. If it meets the boundary at an angle, its direction changes as well, and this bending is called refraction. Entering a denser medium the ray bends toward the normal; entering a rarer one it bends away.

The refractive index n = c/v tells how strongly a medium slows light. Snell's law connects the angles on the two sides of the boundary. The frequency of light stays fixed during refraction; the wavelength shrinks in the denser medium.

• Refractive index n = speed in vacuum / speed in the medium.
• A ray along the normal passes straight through without bending.
• Higher n means slower light and stronger bending.

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