What is Light?


Light is a portion of the electromagnetic spectrum that is visible to humans. Although there are many types of waves, from radio waves to gamma rays, our Sun commonly produces the frequency in the electromagnetic spectrum that is visible light. It’s likely that humans are able to detect this frequency with the retinas in our eyes as a result of evolving on a planet near the Sun that produces these frequencies.  Like all waves in the electromagnetic spectrum, light is a transverse wave and it is distinguished by its wavelength. For example, the color red has a longer wavelength than the color blue.

Electromagnetic spectrum


Wave or Particle?

Despite the known wave properties of light, it also has particle-like properties which confused scientists. In 1887, Heinrich Hertz was the first to observe the photoelectric effect that, amongst other observations of the subatomic world, led to the quantum revolution. Hertz witnessed electrons that were ejected from a metal when light was shone on it. The interesting find that led to quantum physics, and a separation from classical physics, is that the ability to eject the electron is based on the wavelength of the light. Neither the length of time that the light is shone, nor the intensity, determines if the electron is ejected.

Photoelectric effect


For example, a red light shone on a metal surface might not eject an electron. A green light, with a shorter wavelength, might eject the electron. Whereas a blue light, with even shorter wavelength, might eject the electron with greater kinetic energy (velocity) than the green light. The red light could be shone for hours, much brighter than the blue light, and the results would be the same.

In 1905, Albert Einstein recognized that time and intensity were irrelevant in the experiment because light is “quantized” into packets. Prior to Einstein’s paper, light was expected to be a continuous wave. Einstein proved it differently. It appeared more like a particle. It is now called a photon.




In energy wave theory, light is a wave. It does not have mass, as mass is defined as stored energy in standing waves. It is not a particle in wave theory definition, as particles are defined by a formation of wave centers that create standing waves. Rather, it is a transverse wave that is created by a vibrating particle. The vibration is finite, leading to a defined volume for the wave, otherwise known as a photon. The figure below illustrates an electron’s  vibrating motion that creates the wave.



Photon Creation

Electron Vibration – Creating a Transverse Wave


The equations for transverse energy and wavelength contain an initial and final position for a particle that experiences a change in amplitude, which causes motion. However, it does not move from point A to point B like a man walking from his car into his home. Instead, a better analogy is a spring with a marble attached to the end. Stretch the spring and release it and the marble will move back-and-forth as the spring finds its equilibrium. This is the electron’s motion (vibration) before coming to rest. At certain wavelengths, visible light is created. Other vibration speeds lead to other wavelengths from radio waves to gamma rays.



A photon can be absorbed by particles, like the electron, when resonating at the correct frequency.  The explanation below illustrates the photoelectric effect and how the electron’s spin converts transverse wave energy into longitudinal wave energy, increasing the amplitude between the nucleus, forcing the electron away.  For more details, and an explanation of all photon-electron interactions for creation and absorption, refer to the section on photon interactions.

Photoelectric process

Photoelectric Effect Process


Light is indeed a wave. Einstein was also correct and it is a packet, or photon. It is a short-lived packet of transverse wave energy as a result of particle vibration.



Video – What is Light?

The What is Light video below provides an explanation of light and how particle energy can be transferred to a transverse, electromagnetic wave seen as light at certain frequencies.