What Is Light?

The science of optics is a subset of physics that deals with light, its behavior and how light interacts with matter and its environment.

Light is electromagnetic radiation and, owing to the quirks of quantum mechanics, it can behave in a wave-like or particle-like fashion. As an electromagnetic wave, light consists of an oscillating electric and magnetic field, that are perpendicular to each other. The two oscillating fields are perpendicular to the direction of travel, making light an example of a transverse wave.

The Electromagnetic Spectrum

In reality, light is far more than the visible light we are all familiar with. As light is an electromagnetic wave, it extends from low-frequency radio waves through to gamma waves with very high frequencies. The visible light spectrum is a small segment located in the middle of this spectrum between infrared and ultraviolet waves. The visible region of the electromagnetic spectrum is of the range of frequencies of electromagnetic radiation that our eyes are sensitive too.

Figure 1. The electromagnetic spectrum (Courtesy of NASA)

The wavelengths of visible light range from around 400 nm (violet) to 700 nm (red). For comparison, radio waves have wavelengths on the order of 103m whereas the shortest wavelength radiation, gamma waves, have tiny wavelengths on the order of 10-13m.

Science and the Electromagnetic Spectrum

A lot of scientific work and imaging uses special equipment that allows us to record images using wavelengths of light outside of the visible region. One example is the use of X-rays for medical imaging and security detection applications, or infra-red radiation for night vision goggles. Some animals, particularly insects, can naturally see beyond the visible spectrum. For example, bees can see more ultraviolet radiation than humans, and use this to help distinguish flowers.

Speed of Light

While the speed of light can be changed by passing light through materials, such as water, the speed of light in a vacuum is generally considered one of the fundamental physical constants of the universe. Einstein used the speed of light in his famous equation:

E=mc²

This is the basis of the theory of special relativity and is comprised of:

E = Energy in joules

m = mass in kilograms and

c = the speed of light in a vacuum in meters per second.

The reason c is used as the symbol for the speed of light is that it comes from “Celeritas”, a Latin word meaning “swiftness” or “speed”. The speed of light in a vacuum is 299 792 458 m/s. This speed is independent of the wavelength of the light itself, so applies over all regions of the electromagnetic spectrum.

Changing and Modifying Light

Light can be manipulated in many ways and part of the study of optics is developing new tools and equipment to do just that. For example, we can use lenses to focus light on smaller areas or mirrors to change its direction. Prisms can be used to disperse light, making it possible to see the individual colors present in white light.

This article was updated on 11^th March, 2019.