What instrument measures the properties of the electromagnetic spectrum?

Answer 1

There have been many such devices and methods, each best suited to a particular

range of wavelengths. Examples include 'lecher wires' for radio wavelengths, and

many optical devices, including the spectrometer and interferometer, for visible and

near-visible wavelengths.

Physical means aren't used now to measure radio wavelengths, because it's so

much easier and less cumbersome to measure frequencies electronically. However,

any radio transmitting device designed for use at frequencies above a few hundred

MHz has physical components in it that are built with dimensions that only work at

the wavelength where it operates, and if the wavelength (frequency) is not what it's

supposed to be, then the device doesn't work.

It may surprise you to learn that one example of this is the cooking chamber in your

microwave oven. All of it's dimensions are multiples of 6.12 centimeters, which is

1/2 the wavelength of the 2,450 MHz RF power that's used to warm the meatloaf.