Imagine you are on the way back from a mission to Mars. Your party is in two spaceships when your radio link fails: "Houston, we have a problem." You have no mobile phones, no email and no radio link. How are you going to communicate between the two craft? The spaceships have windows so you can wave to each other, but you can't open the window and shout because of the space vacuum... a string-and-two-tin-can telephone will therefore be a bit difficult to arrange.
There is, however, a wonderful solution - use light to transmit the messages instead of wire or string. Your transmitter can be made from a cardboard tube rolled from one of your space manual covers with a piece of aluminium foil from your packed lunch. If you don't have that, you could use the thin plastic from the manual cover and attach a small piece of reflective metal or mirror taped on the end.
Sunlight in space is much brighter than on Earth or Mars, so take the tube near to one of the windows and orientate it so that the reflected spot is directed to the other ship. When you talk in the tube, your sound vibrations cause the foilmirror to vibrate and the distant spot of reflected light jiggles around in accordance with your voice. Amazingly you have captured your voice information in the process - you have put your voice on a light beam.
Every spaceship has solar cells, which convert light into electricity. You have brought along some headphones (for your MP3) and these convert electricity into sound. If you wire a solar cell into headphones, you have a simple device that converts light into sound.
Try this at home or in the school lab. It's as if you have suddenly invented a device that gives you another sense to explore the world. You hear house lights buzz because they are powered from AC mains, the dot of a TV screen can be heard whizzing its way down the screen building up the picture (even if it's a still) and you can even hear the data coming out from the infrared beam from a TV remote control.
Back in space and on board the second spacecraft, they align their solar cell with the spot from your transmitter that you have carefully directed in through their window. They connect computer speakers and set to maximum volume. The solar cell picks up the light converting it into a voltage and the speaker-amp then produces the sound back again... this is the receiver and they hear your voices. If you both have the same set-up, you now have two-way light beam communication.
Your mission was successful and you get home and take a well-deserved holiday. On a beach watching the sun going down, you notice the light glinting off houses and restaurant windows. The sounds in the buildings will slightly vibrate the windows, their reflections beaming off into air, then out into space - voices and music on light beams speeding out at the speed of light.
Just a fantasy? The Mars mission maybe, but light beam communication is real. This is the workshop that Richard Robinson and I are taking out to Sussex schools as part of the run-up to the Brighton Science Festival 2008, which starts tomorrow (February 23) and runs until March 2.
During the workshop, pupils learn a little about light and then have to make their own light beam transmitter from a torch, plastic cup, foil, small mirror, tape and anything else to hand. When they hear their voices coming out from the receiver headphones the look on their faces says it all. Wow
Dr Jonathan Hare runs the creative science centre at the University of Sussex to enable pupils to experiment for themselves