Fish Tank TIR

total internal reflection - critical angle - refractive index

What it shows:
A simple qualitative demonstration of total internal reflection using a laser beam.

How it works:
Using a fish tank suitably doped with a scattering agent (see Setting it Up), a ray of light from a laser beam can probe the water-air boundary of the water surface to be totally internally reflected when the critical incidence angle (48°) is surpassed. The demonstration is confused (or enhanced) by further scattering off the tank's side walls.

figure 1. laser light entering the tank

Setting it up:
We use a 30cm deep 12L Plexiglass tank (but a commercial fish tank is fine), and a 10mW HeNe laser. A couple of drops of milk added and stirred is enough to show the beam clearly with the hall lights down.

Refraction into the tank could also be shown by angling the beam down upon the surface of the water. The beam is invisible in air but turning on the lecture-bench overhead lights a tiny bit allows the audience to see the angle of incidence of the laser. Some smoke (dry ice or a smoke generator) would show up the incident beam. Rating *