Snell’s Law and Total Internal Reflection

Theory

Snell’s law: $n_1\sin {\theta }_1=n_2\sin {\theta }_2$. From glass ($n=1.52$) to air, beyond the critical angle ${\theta }_c=\arcsin (1/n)\approx 41°$ there is no refraction: the light is totally reflected (the principle of optical fibres).

What to observe

• Red laser at 30°: it refracts out (with a faint reflection).
• Green laser at 50°: total internal reflection.

Try this

• Rotate the lasers, keeping them aimed at the centre of the flat face.
• Change the material to diamond (n = 2.42).

Also available in: Lei de Snell e reflexão interna total (Português) · Ley de Snell y reflexión interna total (Español)