Refraction, Snell's Law and Total Internal Reflection

Scenarios

Angle of incidence, i: 50° (or drag the torch)

Medium 1 (top, light starts here)

n_i

Medium 2 (bottom)

n_R

Angle of refraction, R

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Critical angle, sin i_c = n_R / n_i

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Total internal reflection?

Speed of light, v = c / n

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Drag the torch round to change the angle of incidence. Then work through the scenarios in order: air into water, water into air, and past the critical angle.

Snell's law: n_i sin i = n_R sin R. Both angles are measured from the normal, not from the surface.
The refractive index is n = c / v, with c = 3.00 × 10⁸ m s^-1. The dots travelling along the rays are pulses of light: they slow down in the medium with the larger n.
The faint ray reflected at the surface shows that a little light is always reflected. It gets stronger as i nears the critical angle, and beyond the critical angle all the light is reflected.