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.

Comments:
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 *