What is Light?

I apologize for getting a bit academic here, but I’ll try to keep it to a minimum, and in my defense there are only two short, little equations and one graph in this entire article.

Light is an electromagnetic (EM) wave, a combination of electric and magnetic fields oscillating in time with a wavelength of approximately 0.5 µm—the diameter of a human hair is about 100 µm—and a frequency near 6 x 1014 Hz; that’s a 6 with fourteen zeros. 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. And that makes a good point: AM and FM radio waves; UHF and VHF TV signals; microwaves; infrared, visible and ultraviolet light; x-rays, gamma-rays; they’re all part of the electromagnetic spectrum. Furthermore, frequency and wavelength are related through the relationship:

equation relating frequency and wavelength

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.

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 quanta 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.