The waves that accompany light are made up of oscillating, or vibrating, electric and magnetic fields, which are force fields that surround charged particles and influence other charged particles in their vicinity. These electric and magnetic fields change strength and direction at right angles, or perpendicularly, to each other in a plane (vertically and horizontally for instance). The electromagnetic wave formed by these fields travels in a direction perpendicular to the field's strength (coming out of the plane). The relationship between the fields and the wave formed can be understood by imagining a wave in a taut rope. Grasping the rope and moving it up and down simulates the action of a moving charge upon the electric field. It creates a wave that travels along the rope in a direction that is perpendicular to the initial up and down movement.

Because electromagnetic waves are transversethat is, the vibration that creates them is perpendicular to the direction in which they travel, they are similar to waves on a rope or waves traveling on the surface of water. Unlike these waves, however, which require a rope or water, light does not need a medium, or substance, through which to travel. Light from the sun and distant stars reaches the earth by traveling through the vacuum of space.

The waves associated with natural sources of light are irregular, like the water waves in a busy harbor. Scientists think of such waves as being made up of many smooth waves, where the motion is regular and the wave stretches out indefinitely with regularly spaced peaks and valleys. Such regular waves are called monochromatic because they correspond to a single color of light.

figure 9

When light passes through a slit (refers to fig. 9) with a size that is close to the lights wavelength, the light will diffract, or spread out in waves. When light passes through two slits, the waves from one slit will interefere with the waves from the other. Constructive interference occurs when a wave front, or crest, from one wave coincides with a wave front from another, forming a wave with a larger crest. Destructive interference occurs when a wave front of one wave coincides with a trough of another, cancelling each other to produce a smaller wave or no wave at all.

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