8.1 Introduction to Optics
This section looks at waves from a more mathematical point of view. The waves discussed will be light waves, because their very small wavelength means that any diffraction effects are negligible. The result is that they can be considered as travelling in straight lines, which is useful for us in our calculations.
Unfortunately, light waves, or more generally, electromagnetic waves, are still not completely understood even by todays top scientists. In order to explain how light behaves, it has been described as having a combination of both wave and particle behavior. This interesting effect is part of Quantum Theory, and area of Physics that will not be discussed in PhoolProof Physics. For our purposes, however, only the wave-like aspects of light will be discussed.
Imagine looking down on an ocean. What you would see, instead of waves bobbing up and down, would be several lines traveling across a blue expanse. These lines in reality would be the crests of ocean waves. They stretch out in either direction, perpendicular to the movement of the waves. The lines, known by physicists as wavefronts, are common to all transverse waves, though they are usually imaginary. The distance between wavefronts is the distance between the crests of a wave, that is, one wavelength. Longitudinal waves also have wavefronts, which are (usually imaginary) lines that extend along the compressions of the wave.
In optics, however, what is more important for consideration that wavefronts are the imaginary lines that can be drawn perpendicular to them. These are known as wave trains or more often as rays. Rays, like waves, have a source and a direction, and are drawn in optics diagrams as a line with an arrow indicating the direction. By effectively summarizing a series of waves into a line, it is easier in many cases to see how the wave behaves, and to make calculations for its movement.