Electricity, Magnetism, and Light

James Clerk Maxwell demonstrated that the light is made of electric and magnetic fields that change very rapidly. When you walk across a rug in the winter (and there isn't very much water in the air), you can collect electric charges that make your hair stand on end. You can sometimes give other people shocks by moving these electric charges from you to them (and they can shock you by doing the same).

Your hair stands on end because the electric charges create a "force field" that pushes on other charges. We call that force field an electric field because electrical charges make it.

You are probably also familiar with magnets. If you look closely at a magnet, you will find that it always has two "poles" (refrigerator magnets are flat - with one pole on one side of the magnet and the other pole on the other side - so finding the magnet's poles can be confusing). The Earth is also a magnet - with a North Pole and a South Pole. If we suspend a small magnet so that it is free to turn, one of the magnet's poles will turn until it points north. The magnet turns because the Earth's magnetic field creates a force that pushes the magnet to point north and south.

Maxwell's work showed that electricity and magnetism were connected. His equations showed that if electric charges are pushed or pulled, the changes in the speed of the charge create magnetic fields. In the same way, if magnetic fields change, they can create electric fields. This understanding allowed engineers to create electric generators that have big magnets. The generators create flowing electricity by pushing electric charges in wires with magnetic fields created by the magnets.

Maxwell's equations also showed that electric and magnetic fields could work together to make a moving wave. In one part of the wave, there is a strong electric field. In another part of the wave, there is a strong magnetic field. As the electric field dies away, it induces (which is a fancy word for creates) a magnetic field to take its place. When the magnetic field dies away, it creates a new electric field.

This picture shows one way of thinking about electric and magnetic fields in a light ray. The light is moving to the right. The grey lines show the electric field pointing up and down. On the right side of the square box, there is a lot of electric field (many field lines). An electron that encountered this field would be pushed down. About one-third of the way across the box, there are fewer field lines - which means that there is less electric field. An electron there would not feel very much force. In the middle of the box, the electric field is again strong - but at this place, the field points in the opposite direction. An electron there would feel a force pushing it upward.

The black lines show the magnetic field pointing into and out of the figure. The electric and magnetic fields point at right angles to each other. The black circles are like the tips of arrows coming toward us. The 'plus' signs are like the feathers at the end of arrows moving away from us. The magnetic fields push on moving electric charges.

The distance between two parts of a wave that have the same electric field strength is called the wavelength. You can see that distance in the figure, marked with the Greek symbol for lambda, λ. You can explore how light's wavelength is related to the speed of light and how fast the electric and magnetic fields change.