Properties of Waves
As a type of electromagnetic radiation, light falls into the category of transverse waves. With transverse waves, components oscillate perpendicular to the motion of the wave. Anybody who has spent time at the beach, for example, knows all about transverse waves and their perpendicular motion. Water waves are a good analogy for what happens with electromagnetic waves. As a water wave rolls toward the shore, swimmers bob up with its crest and down with its trough. This is what we mean by perpendicular motion.
Not all waves are transverse. With longitudinal waves, the components of the wave oscillate or vibrate in parallel to the wave's direction. As an example, think of a coil spring. Pulling it out and pushing it back causes its components to compress.
Like ocean waves, electromagnetic waves move together in a series. Imagine yourself hovering above the ocean, looking down at the wave. What you might see looks a like a series of ridges in the water (see Figure 3.5). The lighter areas are where waves peak and the darker areas where they fall. This series of waves is called a wave train. The direction of the wave train is indicated by drawing a ray across the waves' peaks.
Figure 3.5 The direction of the wave train is indicated by the three rays.
Look at a wave train in profile and you get a sine wave with certain distinct properties of height, length, frequency, and speed. The height or amplitude of the wave is measured from the wave's peak, or crest, to the axis around which the wave moves. The amplitude is also a measure of the brightness of the pulse. The distance between the successive troughs of the waves is the wavelength. The ability of DWDM systems to use signals of different wavelengths to carry different transmissions has enabled providers to dramatically increase the capacity of their fibers (see Figure 3.6).
Figure 3.6 Viewed in profile, an electromagnetic wave has a sinusoidal form
The number of times a wave oscillates each second, or it's frequency, is measured in hertz (Hz) after Heinrich Hertz, the physicist who discovered radio, not the car rental company. A hertz refers to a complete cyclestarting where the wave begins its rise and fall. The number of times a cycle crosses a particular point in space, which is the inverse of frequency, is called the period.
If two waves arrive at their crests and troughs at the same time, they are said to be in phaseor, to put it another way, waves that are in phase appear symmetrical. Similarly, if two points on a wave are separated by whole measurements of time or of wavelength, they are also said to be in phase (see Figure 3.7).
Figure 3.7 Phase in waves and points.
Another important property of light is its speed. The speed of a wave can be calculated by multiplying frequency and wavelength. Since all electromagnetic waves travel at the speed of light, which in a vacuum is 299,792,458 meters per second or around 300,000 kilometers per second, only frequency or wavelength needs to be known. The constant speed of light yields an inverse relationship between frequency and wavelength. The higher the frequency, the shorter the wavelength.
Actually, referring to an electromagnetic wave as a sinusoidal wave is a simplification. Electromagnetic waves are called as such because they consist of electrical and magnetic fields moving orthogonally, or at right angles, and in phase with one another. Since the two fields normally do not interfere with one another, only the electrical component is shown.