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Refraction demonstration


To demonstrate clearly how refraction works, fill a small aquarium
with water. Pour a little ordinary red ink into the tank and stir.
Arrange a slide projector or a beam from a powerful electric torch
to produce a horizontal, thin slit beam of light. In a dimly lit
room allow the slit beam to hit the water at various angles and note
how the beam bends at a sharp angle as it hits the water. If the
beam strikes at a low angle the light will be totally reflected off
the surface of the water (Angle of total reflection) See how at
another angle the light reflects completely from a side or the
bottom of the aquarium. If the tank is long and narrow, you can
show how at a certain angle the light beam refracts, is reflected
from a surface, and finally emerges at a different angle. At a
certain entrance angle the light will emerge from the tank having
broken into a rainbow of colour; a spectrum akin to fire from a well
cut gem. (angle of refraction). The details seen in that spectrum
shows exactly how the original light was made up. If for instance
you used light from a sodium lamp you would see two bright yellow
lines and a few pale colours instead of the rainbow effect. Now
you’re into spectroscopy.

You can understand now how a good gem cutter can arrange for light
to enter the table of a gem, bounce around inside the stone,
emerging as pinpoints of bright light and the amazing fire of a tiny

Diffraction is a totally different game, so I will keep it out of
this as it will tend to muddy the waters of understanding. Suffice
it to say this is what you see on labradorite, (but not the hairy
variety) (Gor blimey, guv, talks all posh, don’ 'e)

Cheers for now,
JohnB of Mapua, Nelson NZ

Thank you John (Burgess) for that great explanation of refraction.
My children will learn as much (or more) as I will when I set up the
aquarium experiment and play with the light and angles. I now
understand much more about some of the issues cutters face. It would
be wonderful it you could see your way to a short discussion of
diffusion, also.

Sheridan Reed