The distance between parts of a wave?

Answer 1

Wavelength is the longitudinal distance measurement.

Wave 'height' is termed 'amplitude'. But always remember that the horizontal datum can vary. The 'ground state' condition may be non-zero, and thus may be below the wave trough level (as in surface waves). Datum or Zero may also be at the median position, so the 'trough' is a minus value equal to the peak positive value.

Wavelength of moving waves is the relevant constant and physical characteristic, but because it is difficult to measure we use the frequency of the peaks passing the observer. This can cause confusion as frequency is observer frame dependent (i.e. if two observers are moving at different speeds they get different apparent frequencies for the same wavelength).

Frequency is also wave speed dependent, i.e. sound waves of the same length as a light wave will pass an observer much less frequently.

But c = f*lambda is a constant, So in water, the speed of light c reduces to 140,000 miles/sec, wavelength reduces (blue shifts) so the recorded frequency ( the derivative) remains the same.

Frequency is measured in hertz