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Geometric Optics

Refraction & Snell's Law

Light bends as it crosses between materials — and can be trapped entirely inside the denser one.
Incident Refracted Reflected Normal
Light bends toward the normal entering a denser medium.
Snell's law: n₁·sin θ₁ = n₂·sin θ₂  |  total internal reflection if sin θ₁ > n₂/n₁
i About this experiment — click to learn the physics ▼

What you're looking at

A ray of light travels from one transparent material into another — say from air into water. At the boundary it bends. The amount of bending is set by how much each material slows light down, captured by its refractive index n (n = 1 in vacuum, 1.33 in water, 2.42 in diamond).

Snell's law

The incident and refracted angles (both from the normal) are tied together by the two indices:

n₁ · sin θ₁ = n₂ · sin θ₂ the law of refraction
  • Going into a denser medium (n₂ > n₁), light slows and bends toward the normal.
  • Going into a lighter medium (n₂ < n₁), it speeds up and bends away from the normal.
  • Hit head-on (θ₁ = 0) and it doesn't bend at all.

Total internal reflection

When light tries to leave a denser medium for a lighter one, the bending-away grows with angle until, at the critical angle, the refracted ray would lie flat along the surface:

θ_c = arcsin(n₂ / n₁) only when n₁ > n₂

Beyond that angle there's no escape — all the light reflects back inside. This total internal reflection is what guides light down optical fibres and makes diamonds (with their tiny 24.4° critical angle) sparkle. Set the top medium denser than the bottom and sweep the angle past θ_c to see the refracted ray vanish.

Things to try

Air → diamond: watch how sharply the ray bends toward the normal. Then flip to diamond → air and raise the angle — the refracted ray swings flat and disappears at just 24°. A little light always reflects too (the green ray), even below the critical angle.