Light (Optics): Definition, Units & Sources (w/ Spectrum)

Understanding light allows us to understand how we see, perceive color and even correct our vision with lenses. The field of ​optics​ refers to the study of light.

What Is Light?

In everyday speech, the word "light" often really means ​visible light​, which is the type perceived by the human eye. However, light comes in many other forms, the vast majority of which humans cannot see.

The source of all light is electromagnetism, the interplay of electric and magnetic fields that permeate space. ​Light waves​ are a form of ​electromagnetic radiation​; the terms are interchangeable. Specifically, electromagnetic waves are self-propagating oscillations in electric and magnetic fields.

In other words, light is a vibration in an electromagnetic field. It passes through space as a wave.


  • The speed of light in a vacuum is 3 × 108 m/s, the fastest speed in the universe!

It is a unique and bizarre feature of our existence that nothing travels faster than light. And although all light, whether visible or not, travels at the same speed, when it encounters ​matter​, it slows down. Because light interacts with matter (which doesn't exist in a vacuum), the denser the matter, the slower it travels.

Light's interactions with matter hint at another of its important characteristics: its particle nature. One of the strangest phenomena in the universe, light is actually two things at once: a wave and a particle. This ​wave-particle duality​ makes studying light somewhat dependent on context.

At times, physicists find it most helpful to think of light as a wave, applying to it much of the same mathematics and properties that describe sound waves and other mechanical waves. In other cases, modeling light as a particle is more appropriate, for instance when considering its relationship to atomic energy levels or the path it will take as it reflects off a mirror.

The Electromagnetic Spectrum

If all light, visible or not, is technically the same thing – electromagnetic radiation – what distinguishes one type from another? Its wave properties.

Electromagnetic waves exist in a spectrum of different wavelengths and frequencies. As a wave, light's speed follows the wave speed equation, where the speed is equal to the product of wavelength and frequency:

v-\lambda f

In this equation, ​v​ is wave velocity in meters per second (m/s), ​λ​ is wavelength in meters (m) and ​f​ is frequency in hertz (Hz).

In the case of light, this can be rewritten with the variable ​c​ for the speed of light in a vacuum:

c=\lambda f


  • c​ is a special variable representing the speed of light in a vacuum. In other media (materials), light's speed can be expressed as a fraction of ​c.

This relationship implies that light can have any combination of wavelength or frequency, so long as the values are inversely proportional and their product equals ​c​. In other words, light can have a ​large​ frequency and a ​small​ wavelength, or vice versa.

At different wavelengths and frequencies, light has different properties. So, scientists have divided up the electromagnetic spectrum into segments representing these properties. For example, very high frequencies of electromagnetic radiation, like ultraviolet rays, X-rays or gamma rays, are very energetic – enough to penetrate and harm body tissues. Others, like radio waves, have very low frequencies but high wavelengths, and they pass through bodies unimpeded all the time. (Yes, the radio signal carrying your favorite DJ's tracks through the air to your device is a form of electromagnetic radiation – light!)

The forms of electromagnetic radiation from longer wavelengths/lower frequencies/low energy to shorter wavelengths/higher frequencies/high energy are:

  • Radio waves
  • Microwaves
  • Infrared waves
  • Visible light
  • Ultraviolet light
  • X-rays
  • Gamma rays

[insert diagram of EM spectrum]

The Visible Spectrum

The visible light spectrum spans wavelengths from 380-750 nanometers (1 nanometer equals 10-9 meters – one-billionth of a meter, or about the diameter of a hydrogen atom). This part of the electromagnetic spectrum includes all the colors of the rainbow – red, orange, yellow, green, blue, indigo and violet – that are visible to the eye.

[Include a diagram with a blowout of the visible spectrum]

Because red has the longest wavelength of the visible colors, it also has the smallest frequency and thus the lowest energy. The opposite is true for blues and violets. Because the energy of the colors is not the same, neither is their temperature. In fact, the measurement of these temperature differences in visible light led to the discovery of the existence of other light ​invisible​ to humans.

In 1800, Sir Frederick William Herschel devised an experiment to measure the difference in temperatures for different colors of sunlight that he separated using a prism. While he indeed found different temperatures in different color regions, he was surprised to see the hottest temperature of all recorded on the thermometer just beyond the red, where there appeared to be no light at all. This was the first evidence that more light existed than humans could see. He named the light in this region ​infrared​, which translates directly to "below red."

White light, usually what a standards light bulb gives off, is a combination of all the colors. Black, in contrast, is the ​absence​ of any light – not really a color at all!

Wave Fronts and Rays

Optics engineers and scientists consider light in two different ways when determining how it will bounce, combine and focus. Both descriptions are needed to predict the final intensity and location of light as it focuses through lenses or mirrors.

In one case, opticians look at light as series of ​transverse wave fronts​, which are repeating sinusoidal or S-shaped waves with crests and troughs. This is the ​physical optics​ approach, as it uses the wave nature of light to understand how light interacts with itself and leads to patterns of interference, the same way that waves in water can intensify or cancel one another out.

Physical optics began after 1801 when Thomas Young discovered light's wave properties. It helps to explain the workings of such optical instruments as diffraction gratings, which separate the spectrum of light into its component wavelengths, and polarization lenses, which block certain wavelengths.

The other way to think of light is as a ​ray​, a beam following a straight-line path. A ray is drawn as a straight line emanating from a light source and indicating the direction in which light travels. Expressing light as a ray is useful in ​geometric optics​, which relates more to the particle nature of light.

Drawing ray diagrams showing the path of light is critical to designing such light-focusing tools as lenses, prisms, microscopes, telescopes and cameras. Geometric optics has been around for longer than physical optics – by 1600, the era of Sir Isaac Newton, corrective lenses for vision were commonplace.

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