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# What is Wavelength?

Wavelength is a physical quantity noted with a Greek letter $$\lambda$$ ("lamda"). The wavelength can only be assigned to periodic waves. Its unit is meters (m).

For a sinusoidal wave, the wavelength is the distance between two crests or two troughs.

In general, the wavelength is the smallest distance between two points of the same phase. More simply, the wavelength is the length of ONE section of the wave that repeats.

How can the wavelength be determined theoretically?
The value $$\lambda$$ can be determined from the snapshot (e.g. with a photo) of a wave (see illustration 1 and 2). In the case of a sinusoidal oscillation, the distance from one wave crest to the neighboring wave crest must be measured. Or from the wave trough to the neighboring wave trough.

The wavelength refers to the length of a wave segment and the period refers to the duration of a wave segment. That is, to determine the wavelength, the wave must be plotted in an amplitude-POSITION diagram. On the other hand, to determine the period, the wave must be plotted in an amplitude-TIME diagram.

The wavelength $$\lambda$$ is related to the frequency $$f$$ via the phase velocity $$v_{\text p}$$ of the wave:

Table : Examples for wavelengths
Type Wavelength
Green light $$546 \cdot 10^{-9} \, \mathrm{m}$$
Blue light $$435 \cdot 10^{-9} \, \mathrm{m}$$
Ultraviolet light $$365 \cdot 10^{-9} \, \mathrm{m}$$
Sound $$4 \, \mathrm{m}$$ to $$0.003 \, \mathrm{m}$$
Radio $$10 \, \mathrm{m}$$ to $$20 \, \mathrm{m}$$