What is Light?

by

J. L. Doty

Light is an electromagnetic (EM) wave, a combination of electric and magnetic fields oscillating in time and traveling through space. It has a wavelength of approximately 0.5 µm[1] and a frequency in the neighborhood of 6 x 1014 Hz; that’s a 6 with fourteen zeros. To put that into perspective, a typical human hair has a diameter of about 150 µm. Some of us who bend light like to snobbishly remind our microwave colleagues that they’re working in the low frequency end of the spectrum.

The figure below illustrates a large portion of the electromagnetic spectrum:

It contains scales for both frequency and wavelength. For example, AM radio waves have a frequency near 1 MHz[2] and a wavelength near 300 meters. The table below lists wavelength and frequency for certain types of well-known signals.

Type	Frequency	Wavelength
AM Radio	1 MHz	300 m
FM Radio	100 MHz	3 m
Wifi	2–6 GHz	15–5 cm
Microwave	300 MHz - 300 GHz	1 m - 1 mm
Green Light	5.54 x 1014	0.55 µm

Not listed are UHF and VHF TV, cell phone, cordless phone, CB, cosmic rays, etc.; they’re all part of the electromagnetic spectrum. Furthermore, frequency and wavelength are related through the relationship:

where ν is the frequency, c is the velocity of light (approximately 3 x 108 m/s) and λ is the wavelength.

Along with its wave nature, EM radiation also exhibits a particle (quantum, plural: quanta) nature, as if it were a stream of discrete particles, each carrying a small amount of the beam energy. This is true of any form of radiation like, for example, acoustic waves. EM quanta are called photons, while acoustic quanta are called phonons, and the energy of a quantum is proportional to the frequency (ν). Keep in mind that the relationship between frequency and wavelength related in the above equation holds throughout the entire spectrum, so long wavelengths are associated with low frequencies and weak photons, while short wavelengths are associated with high frequencies and strong photons.

It’s interesting to note that the visible portion of the spectrum is a nice crossover region. At lower frequencies (radio and microwave) the wavelengths are long enough to be easily observed, while the photon energy is so weak it’s almost impossible to detect experimentally. At the high frequency end of the spectrum (x-ray, gamma-ray) wavelengths are so short it’s difficult to observe them, but an individual photon chugs along with the energy of a freight train. X-rays, gamma rays and cosmic rays are referred to as ionizing radiation because their individual photons have enough energy to split an electron off of the molecules in air, ionizing them.

Light is right in the middle. The wavelengths are just long enough, and the photon energy is just high enough, to observe both. Regardless, any phenomena can be explained using either quantum or wave theory, but diffraction , which is discussed elsewhere, is so much more elegantly explained by the wave nature of light, whereas a quantum explanation, while valid, is somewhat inelegant and forced. On the other hand, the way in which gamma-ray photons bombard and damage molecules is clearly much more easily explained by its particle nature. And optical amplification, discussed in What is a Laser?, is almost impossible to explain without the quantum nature of light.

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[1] 1.0 µm = one millionth of a meter

[2] The Institute of Electrical and Electronic Engineers long ago chose to honor 19th century physicist Heinrich Hertz by using his last name (abbreviated Hz) as the symbol for cycles per second.