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Basics of waves

What is a wave, actually? According to Wikipedia, a wave is “a disturbance or oscillation (of a physical quantity) that travels through matter or space, accompanied by a transfer of energy”[1]. Sounds complicated? It doesn’t have to be. In this article the very basics of waves will be explained.

The structure of a wave

A wave’s basic structure is always the same: It has a peak and a trough, and a certain distance from peak to peak or from trough to trough. A wave’s oscillation is determined by means of its amplitude.

It expresses how high the wave’s peak is (or how deep its trough) in relation to its baseline; the amplitude is a measure of the wave’s maximal displacement. The distance from one peak or trough to the next is called wavelength. It is denoted with the Greek letter lambda (λ). The greater the wavelength, the lower the wave’s peak.

So, what are waves? Peaks and troughs with a specific height or depth and a certain distance from each other that travel through space.

The frequency of waves

There is a third important property for characterizing a wave: its frequency. It is the reason why light is blue, yellow or red, or why a bass sounds different to a flute. The frequency is the number of a wave’s occurrence per second; its unit is hertz (Hz).

The relation of sound and frequency is easily explained by the example of a guitar string: The higher a wave’s frequency – its oscillations per second – the higher the sound it produces. The closer the hand on the guitar’s neck travels towards the hand hitting the strings, the more the wavelength is shortened. But how does that change the pitch of the sound? It is because wavelength and frequency are related: The shorter the wavelength the higher the frequency and thus the pitch of the sound: The shortened string can vibrate more often in the same time period.

The same goes for a plastic ruler placed on the edge of a desk and made to oscillate. The longer the length of the ruler placed beyond the edge, the longer the wavelength and the lower its frequency will be and the less oscillation it will produce in a given time. When the length of the ruler that runs beyond the edge is reduced, the wavelength of its oscillation will decrease and the frequency will increase, creating a higher pitch.

Wavelength and frequency are related by the wave’s propagation velocity. The propagation velocity in turn depends on the medium through which the wave is travelling. This is why waves can tell us something about the material through which they are travelling. Many analytical methods make use of the interaction of waves and matter.

Waves and analytical technology

Analytical instruments that can create and measure waves and oscillations include density meters, refractometers and CO2 analyzers. Microwave synthesis devices and polarimeters also make use of waves.