Toys and Physics – Light

4.1 Luminous stars

What happens
These stars are made of a luminous material. They can be stuck on your bedroom ceiling to make constellations.

Theory and teaching notes
These are made of a fluorescent material. Illuminating them with ultra violet light will make them glow all the better in the dark. (Take sensible precautions if using an ultra violet lamp).

4.2 Kaleidoscope

What happens
When you look through the kaleidoscope you see multiple images of the view.

Theory and teaching notes
There are two sorts of kaleidoscope – the one shown in the photograph has a face of multiple prisms – giving many images. The other has two mirrors inclined to each other at a small angle. The multiple images in this case are formed by multiple reflections in the mirrors.

4.3 Laser

What happens
A small spot of light is formed on a screen
DANGER: BE CAREFUL OF THE BEAM BOTH DIRECTLY AND BY REFLECTION EVEN FROM A LASER POINTER.

Theory and teaching notes
The semiconductor laser is sufficient to send a fine beam across the lab. It is very useful as a demonstration instrument for interference and diffraction.

4.4 Fibre optic lamp

What happens
When you switch the lamp on light the ends of the plastic fibres glow.

Theory and teaching notes
The fibre optics lamp (or torch) uses a spray of fine plastic fibres to demonstrate the path of light along a fibre from a central light source. Some versions even have lamps that change colour. (Nothing to do with fibre optics – just a filter passing over the lamp).

4.5 Lava lamp

What happens
When the lamp is on large globules of material slowly rise and fall within the liquid.

Theory and teaching notes
When the lamp is switched on the lava lamp heat up. This changes the density of both the liquid and the material within it. The surface tension of the material makes it form spheres and these move up and down as their density changes are slightly different form those of the liquid.

4.6 Sunset

What happens
A beam of light from a torch or a projector is shone through a tank of water to which a small amount of milk has been added (For a tank 25cm x 35cm x 40cm you will need about half one of the small containers of milk served with coffee in a cafe). The water/milk mixture glows blue at the end nearest the projector and this slowly turns to red further along the tank.

Theory and teaching notes
As the light passes through the tank it is scattered by the particles of fat in the milk – the blue light being scattered first. The result is that the water in the end of the tank nearest the projector looks bluish while that at the other end of the tank has a reddish hue – just like a sunset.
A few drops of dettol can be used instead of the milk.

4.7 Glowing putty

What happens
The putty glows in the dark.

Theory and teaching notes
When ultra violet light falls on the putty some of the energy is absorbed and later re-emitted in the visible range. Since the putty can be made into a ball which also bounces it is great fun to use in a darkened lab. Sunlight works as a weak source of ultra violet and so there is no need to use an ultra violet lamp.

4.8 Ball in water

What happens
The clear plastic ball has a flat coloured picture fixed inside. When you put the ball in water the ball seems to stay the same size while the picture suspended inside it gets smaller!

Theory and teaching notes
When it is immersed in water there is a concave lens effect since the ball is full of air. The water-plastic-air surface is concave. This therefore makes the object inside the ball look smaller.

4.9 The Mirage machine

What happens
You reach down to try and pick up the toy animal or car and find that although you can see it the object you will find that you can't touch it!

Theory and teaching notes
What you actually see is the image of the object formed in space by the curved interior of the mirror box. This real image appears to be sitting on the top of the box.