Monthly Archives: July 2014

The invisible rod

Challenge: How can you make a quartz rod invisible with some water, sugar and a beaker?

Answer: Snell’s Law

If we take something that is typically transparent (i.e. the quartz rod) you can normally see it quite clearly when placed in a liquid, by the way in which light is bent as it passes through.

Snell's lawThis ‘bending’ of light – refraction – can be described by Snell’s law:


where ‘n’ is the refractive index of the material.

So you might imagine that if we can change the rod, or the liquid itself, so that light entering from behind the beaker does not refract further on entering the quartz rod, we can effectively make the quartz rod invisible. To do this we want to match up the refractive indices (n_{1}, n_2).

With water, as you add more and more sugar the refractive index increases, until finally it approaches that of quartz ~1.46.

invisible rod2

Homopolar motors

A simple motor that doesn’t need commutators or brushes, only:
– An AA battery
– Some disc-shaped strong magnets
– Some wire and wire cutters and/or pliers
Homopolar motor

The aim is to bend the wire in a roughly heart shaped design so that the top just touches the + end of the battery and the bottom curls around the base (i.e. magnet). It needs to be close enough so that a current will flow but not so close that friction stops it from moving.

When a current flows through the wire a force is exerted on it due to the magnetic field of the magnet at the bottom of the battery. (The direction of this can be determined using Fleming’s left hand rule.) This produces a torque on the wire that results in it rotating about the battery: in the example above the wire would rotate clockwise.