The Photoelectric Effect

Philippa - This week I've been finding out how my do-dah works. Maybe you call it a flicker, but maybe for simplicity I should call it a remote control. Here's Dr Julian Allwood from the Institute for Manufacturing at Cambridge University.

Julian - What happens when you press a button on your remote control is that a light flashes, sending a code in the direction you're pointing. The receiver in the object you are trying to control is waiting and looking for light of one particular frequency, which is the frequency given out by your remote control. Once it sees that frequency, it watches to see a series of 'ons' and 'offs' and works out what that signal was meant to communicate.

Philippa - I've never seen light coming out of my remote control.

Julian - Good comment. The light is usually in the infra-red frequency, which is outside the range of your eyes.

Philippa - How on earth can there be light you can't see? Let's ask Dr Lucy Green from the physics and astronomy department at Cardiff University.

Lucy - Normally when we talk about light, we use it to describe light that our eyes can pick up, including all the colours in the rainbow. In reality, there is a much wider spectrum that we can't pick up with our eyes. This extends from very energetic sources like gamma rays, x - rays and UV light, and then through visible light. After that you move in a region of lower energy waves like the infra-red and radio waves.

Philippa - So infra-red light is just one part of the electromagnetic spectrum.

Julian - You could prove to yourself that it was light because when you use a TV remote you have to point it in the general direction of the TV. If you wanted to check that it was light, you could put a mirror facing the television and fire the remote away from the television. You could then use the mirror to reflect the signal back towards your TV.

Philippa - So we've got a code coming out of the remote control made from pulses of invisible light. If we can't see it, how does the TV see it?

Lucy - The TV picks up the signal by using something that's sensitive to light, or in other words, it uses the photoelectric effect. The light falls on the sensitive detector and electrons are knocked out of a piece of material, which causes a current to flow.

Philippa - Is this the same as in the solar panels of a calculator?

Lucy - Yes, it's exactly the same thing. It's a way of turning light into electricity.

Julian - That's rather a difficult thing to understand because it depends on the behaviour of a semiconductor. Pieces of metal are very good at conducting electricity and heat, while glass does not conduct heat or electricity at all. So a semiconductor is a material that will conduct electricity under some circumstances. They are used as the basic material for transistors, and they are also the basis of light sensors, which are called photodiodes. Some semiconductors exposed to certain frequencies of light will transmit a small current.

Philippa - How does that work?

Lucy - In the classical sense, people thought of light as a wave propagating along through space. They thought that the more light you shone onto a plate, the higher the kinetic energy the electrons should have. This isn't what we see. To change the energy of the electrons being knocked off, you need to change the colour (or frequency) of the light. The more blue the light is, the more energy the light has. In order to be able to understand the photoelectric effect, you can't think of light as a wave. You have to think about it as tiny packets of energy called photons. Blue light has photons of higher energy than the red light, so blue light would knock electrons off with more kinetic energy than a red light would. Einstein worked out how to use this effect, and it helped us to be able to apply the effect in the everyday world.

Philippa - The TV has received the light. What next?

Julian - What the TV does is to amplify this small current and creates enough of a signal so that it can tell whether it is a one or a zero. From that it can build up a series of instructions which it can respond to.

Philippa - How do those all-in-one remote controls work? How does it know when it should be working the TV, the hi-fi or the garage doors?

Julian - In that case, the devices have all agreed an international code. For example, the video has a special code that identifies itself, and therefore if the remote is talking to the video, it starts by saying that code so the video knows to listen and the garage door knows to take no notice.

Philippa - Thanks to Einstein's understanding of photons, I can spend the entire evening sitting on the sofa.

Lucy - Without Einstein you would have to get out of your chair and change the channel yourself!