How is wavelength measure?

Answer 1

A wavelength is a measure of the distance between two consecutive peaks (or troughs) of a wave. For electromagnetic radiation, the wavelengths range from 10^-11 to 10^3 metres - a factor of 100 trillion. There is, therefore, no single instrument which can be used to measure across such a range.

Answer 2

Scroll down to related links and look at "Acoustic waves or sound waves in air and Radio and light waves in a vacuum".
From the relation λ = c/f between wavelength λ, speed c, and frequency f, the wavelength of a wave motion can be calculated if the speed is known and its frequency is measured. The ease and accuracy of electronic counting and timing make frequency measurement the most precise of all physical measurements. This method of wavelength determination is thus one of the most accurate, but only if the speed (phase velocity) is known. In free space the speed of an electromagnetic wave c0 is, through the 1983 definition of the meter, fixed at exactly 299,792,458 m/s (186,282.397 mi/s), or roughly 300,000 km/s. Unless otherwise specified, it is general practice to quote the wavelength of an electromagnetic wave as the free-space value λ0, given by the equation below. ()