The right wavelength
Your students may not think that the range of electromagnetic radiation from gamma rays to radio waves that we call the electromagnetic spectrum is the most riveting part of the science curriculum. Apart from anything else most of the electromagnetic spectrum is invisible so it can be a challenge to explain the relevance of some of the things that are going on.
But encourage your students to imagine how the world would look if our eyes were sensitive to all these waves. Radio waves carrying text messages and music would darken the sky. The kitchen would be filled with information every time you turned on the microwave. In fact, it is probably just as well that we can see only a small proportion of the spectrum - the visible light.
Light, radio waves, X-rays and the microwaves that cook your food are all basically the same type of radiation - the only difference is the wavelength, frequency and energy of the waves.
Ask your students if they have ever noticed how moving around a room can affect radio reception? Why? Well take Radio 1 as an example, broadcasting at a frequency of 98.6MHz. It's only a simple mathematical calculation to discover that the waves carrying the music from Radio 1 are about three metres long.
Most radios receive a signal from an aerial attached to them. This picks up not just the signal sent by the transmitter, but also the radio waves that bounce around the room. The original broadcasted waves can interfere with the reflected waves and sometimes cancel each other out. This gives you a varying strength signal that can be altered by simply moving around the room. Ask your students to work out how much they need to move to affect the signal of Radio 1. Would it vary with different sized rooms?
Microwaves are another type of electromagnetic radiation, but again the same rules of interference help us explain a lot about how they work. A microwave oven makes electromagnetic waves that interact with water molecules inside the food to heat it up. The water molecules in the food have oppositely charged ends.
The microwaves push and pull on the charged ends of the water molecules, causing them to move back and forth very quickly. The frequency of a microwave is about 2.45 gigahertz which means that the water molecules move back and forth 2,450,000,000 times a second. As they move, they rub against each other producing friction, which produces heat.
It is said that microwaves can't heat an ice cube, which is partly true.
The heating process relies on the fact that the water has to be free to move. If the molecules are bound up too tightly in the ice they can't move, so an ice cube can be very difficult to heat using a microwave. See how long it would take to melt an ice cube on different settings. If you are trying to heat frozen food it often helps to add a bit of water on top of the food. Once that gets warm, it helps melt the ice and then the usual agitation of the molecules can begin.
Incidentally, when you use the defrost setting, your microwave is switching on and off rather than giving a continuous level of heating. It does this because after a burst of water molecule vibration, a short break will allow the small amount of heat within the food to spread to the surrounding areas. This slowly frees up more water molecules from their icy trap, so the next burst of microwaves can be more effective.
By knowing the frequency of the microwaves, we can calculate that their wavelength is about 12cm. People often ask how safe it is to stand in front of a microwave or look into it. The grid across the door of the microwave has gaps that are much smaller than 12cm so, as far as the microwaves are concerned it is a solid, impassable surface. We can see through the grid because light waves have a wavelength that is much shorter so they easily pass through, allowing us to see what the food is doing.
One reason that food sometimes cooks unevenly in a microwave is also down to a type of wave interference. Because the waves cannot escape from the microwave box, they are effectively fixed at the sides. Think of the waves like a skipping rope - it is fixed at both ends so the rope doesn't vibrate at these points. This is what happens in a microwave. The metal casing is the fixed point in the vibration, so the microwaves set up standing waves.
This means that at some points in the oven there is lots of vibration going on and at other points there is very little. This gives a microwave a very specific pattern of hotspots which is why you need a rotating table to cook the food more evenly. You can investigate where the hotspots in your microwave are by taking out the rotating tray and putting in a stationary plastic tray of small marshmallows arranged in a grid. After a few seconds you will see that some marshmallows have completely melted while others remain unaffected. Why not ask your students to plot a chart of their microwave hotspots? Perhaps different-sized ovens have different patterns of hotspots.
Many other everyday items use electromagnetic waves: the TV remote control; your mobile phone; and CCTV cameras that can see in the dark. By understanding a little about the way these invisible waves interfere, we can discover much about how these things work. And they say physics only happens in the lab.
Wendy Sadler is an Institute of Physics lecturer
This article is based on an interactive show for secondary physicswww.science-made-simple.co.uk
A virtual marshmallow experiment that won't get your microwave sticky
A good introductory page about the electromagnetic spectrum
www.newscientist.comlastwordSearch for "radio" for more information about using your body to interfere with radio waves