What is light?
In physics, light is the smallest quantity of energy that can be transported. A photon, an elementary particle without a real size, cannot be split, only created or destroyed. Light also has a wave-particle duality, being kind of particle and a wave at the same time. However, in ordinary usages, when we say light, we actually mean visible light, which is a very tiny part of the electromagnetic (“EM”) spectrum, that are the range of all types of EM radiation. Radiation is energy that travels and spreads out as it goes. Radio waves that come from a radio station is another type of EM radiation. All types of such electromagnetic radiation travel 299,792,458 meters per second in a vacuum, also known as the speed of light (commonly denoted as “c”).
What is EM radiation?
Electromagnetic radiation includes an enormous range of wavelengths and frequencies and carries different level of energy. The EM spectrum is generally divided into eight regions, in order of decreasing wavelength and increasing energy and frequency, they are commonly named:
- radio waves,
- Terahertz radiation,
- infrared (IR),
- visible light,
- ultraviolet (UV),
- X-rays, and
- Gamma rays.
Gamma rays have the smallest wavelength because they are the highest energy photons; radio wave have the largest wavelength because they are the lowest energy photons. How many energy difference among the ends of the electromagnetic spectrum? Electron-volt, or eV, is a unit of energy in physics, widely used in solid state, atomic, nuclear and particle physics. In astronomy, it is widely used to describe the energy level of light or EM radiations.
Figure 1: Electromagnetic Spectrum
The Planck-Einstein relation (E = hf, where h is known as Planck’s constant) connects the particulate photon energy (E) with its associated wave length (f). As mentioned above, all types of EM wave are at the same speed of c. Therefore, the particulate photon energy has an inversed relationship with its associated wavelength. Now, let us take radio wave and gamma rays as examples. The biggest long wavelength of radio wave can span from 10,000,000m to 100,000,000m. Most gamma rays are under 10 picometers (denoted as “pm”, one picometer is 1 x 10-12, i.e. 1/1000 of nanometer), which is far smaller than a hydrogen atom. By applying the Planck-Einstein relationship, we can depict a huge energy difference between very long wavelength radio waves and gamma rays i.e., of 1020 (hundred million trillion or hundred quintillion) times!
What is visible light?
What makes visible light special? Um…our eyes make them special. Human eyes are also evolved to capture visible light which carries the information around us. However, not all animal eyes were evolved in the same way, for example, bird eyes can recognize ultraviolet rays which is generally invisible by human eyes. Furthermore, bug eyes like bee eyes can recognize ultraviolet rays as well.
Visible light is not white. A beam of white light will be dispersed into a visible light spectrum as below. Rainbow is a natural phenomenon happened after raining.
Figure 2: Dispersion of white light through a prism
Figure 3: Rainbow after a rainy day
Such visible light spectrum are EM radiations of wavelength from 380 to 750 nanometers.
Figure 4: Visible light spectrum
By applying the Planck-Einstein relationship, we know that gamma rays is 100,000 times powerful than the visible lights.
In short, light includes all electromagnetic radiations travelling at the speed of light, c, in vacuum. All those electromagnetic radiations form a continuous electromagnetic spectrum. Light, in our ordinary use, generally refers to visible light, which is just a tiny part of electromagnetic spectrum. As a matter of fact, solar light from the Sun includes not only the visible light but also other electromagnetic spectrum, like ultraviolet rays. Even the artificial light sources emitting electromagnetic radiations other than visible light, for example, tanning lamp emits the ultraviolet rays which make our skin tanning. However, as we explained above, some of the electromagnetic radiations are much more powerful than visible light, and we should avoid exposing in those high energy electromagnetic radiations.