Waves are ubiquitous in the study of physics, and we are intuitively familiar with them from a very early age, but what are they really?
A wave is the propagation of energy through a medium. The propagation part means that energy is transferred from one position in time and space to another position. The medium can be some form of matter, like water in the case of ripples, or air in the case of sound, or it can be a vacuum, such as in the case of light. The medium itself is often not permanently disturbed by the passing of the wave, and will return to its original position once the wave has passed. Physicists still aren't entirely sure why energy tends to travel in this manner.
A wave crest marks the top of a wave, and a wave trough marks the bottom of a wave. A wavelength is the complete stretch of a wave from crest to crest or trough to trough. The period of the wave is the length of time it takes for one wavelength to pass a certain point in space. In contrast, a wave frequency is the number of wavelengths that pass a certain point in space in a particular given time. The amplitude of a wave is the distance from the center of the wave to a crest, or the center of the wave to the trough. The motion of the wave changing from crest to trough is called oscillation.
A wave can be longitudinal or transverse, or a combination of both. A longitudinal wave oscillates parallel to the forward motion of the wave. Sound travels in longitudinal waves. A transverse wave oscillates perpendicular to the forward motion of the wave. Electromagnetism travels in transverse waves. Water ripples typically travel as a combination of longitudinal and transverse waves.
A wave can change direction in one of two ways. It can bounce or reflect off another medium, such as a beach in the case of water, or a mirror in the case of light. A wave can also refract as it enters another medium. Sound that travels through a wall sounds very different than sound that travels only through air. The sound waves have gotten bent as they traveled from air to wall.
Waves can also interfere with each other, which can produce chaotic, complex behavior. Crests and crests intensify each other where they cross, troughs and troughs deepen, but crests and troughs will neutralize each other. Two waves passing each other will be affected where they cross, but maintain their energy once they pass.
