What is the difference between wavelength and amplitude?

Answer 1

Wavelength - the distance from one wave crest or trough to another wave crest or trough.

Amplitude - the distance from the median point or "middle" of the wave straight up to a crest (a maximum) or straight down to a trough (or minimum), which is the peak amplitude; or the distance from a trough straight up to a crest, or a crest straight down to a trough, called peak-to-peak amplitude. A more general definition for amplitude might be the "height" of the wave. Note that wavelength is a function of the frequency of a given wave or "signal" and the speed at which that wave travels. One way to look at things is to throw a rock into a pond and create a wave. The wave would travel outward from the point where the rock struck the water, as you may have guessed. Picture the wave forming in slow motion. As the wave was going through its complete cycle, the wave would be moving away from the point of origin. As the wave goes through one complete cycle, the wave is still moving away. And by the time one complete cycle has occurred, we have only to measure the distance that, say, a crest traveled back to the next crest to determine the wavelength. Think it through and it will lock in.

Answer 2

The wavelength is the distance from one peak (or trough) of the wave to the next peak (or trough). The amplitude is the distance from the mean value to the top of the peak (or the bottom of the trough). Alternatively, the amplitude is one half of the distance between the height of peak to the depth of the trough.

How you measure these depends on the nature of the waves.