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Frequency (article)

Examples

Electromagnetic radiation

Main article: Electromagnetic radiation

Radio waves and microwaves

Radio waves and microwaves are electromagnetic waves, consisting of spatially orthogonal electric and magnetic fields oscillating in both time and space with a relative phase of 90°, traveling through space. The peak electric field occurs where the magnetic field is zero but changing at maximal rate, and vice versa. The wavelengths are in the range of many meters to a few millimeters, corresponding to frequencies that can be directly measured in the time domain by observing the electrical signals induced in an antenna detecting the waves.

Light

Main article: Light

Visible light is also electromagnetic radiation with much shorter wavelength, in the 400 to 700 nm range. In scientific observations, it is detected as a stream of massless elementary particles called photons, each with an energy conventionally in the range 2.9 to 4.6 electronvolts. (At low light intensities, these particles can be individually counted by a sufficiently sensitive photodetector.) In common with other subatomic particles, even massive examples such as cold neutrons, streams of multiple photons arrive at detectors in spatial distributions that reflect wave-like propagation, displaying interference and diffraction. Hence a wavelength associated with any particle of such type can be directly measured using an interferometer. This applies also to massive particles such as neutrons.

From the measured wavelength and speed of photons, what is conventionally called a "frequency" can be calculated in the usual way:

$\displaystyle f=c/\lambda$

where c is the speed of light (c in a vacuum, or less in other media), f is the frequency and λ is the wavelength.

Regarding the interpretation of these quantities, Feynman provided the following advice:

We cannot say whether light is particle or wave. This is not an either/or situation; light seems to be both particles and waves and thus is probably neither. 

Visible light shares that duality with, e.g., cold neutrons, the "frequency" of which is rarely mentioned.