What Is Light?

In short, What is Light?

Light is a common word used to refer to types of electromagnetic radiation located in the spectral region from 380nm to 700nm. In this area, people can completely see light with the naked eye.

Energy, mass, and momentum

The energy of a photon with wavelength λ is denoted as hc/λ, where:
– H is Planck’s constant.
– C is the speed of light in a vacuum
Since the rest mass of the photon is absent, the particle’s momentum will be equal to the particle energy divided by the speed of light.

Light's Interaction with Matter



When traveling through a vacuum, light moves in a straight line, but what happens when it interacts with matter? All matter absorbs and reflects light, with the material’s absorptivity determining the amount absorbed and the amount reflected dictated by an object’s albedo.



Light absorption is the process in which the energy from light is stored within a substance’s atoms. The light energy that comes into contact with the object is transferred to electrons within the material, where it is stored in a manner similar to how energy is stored in a stretched spring. The material's atoms are then referred to as being in an excited state. Eventually, the excited electrons return to their lowest energy state or ground state. The excess energy is sometimes released as invisible infrared, but can also be released as visible light through light reflection. Other times, the stored energy is released as random motion or kinetic energy, which is what we typically think of as what happens when light is absorbed.



Light reflection is similar to light absorption in that, during the process of reflection, the electrons within the material are affected by the incoming light. In response, however, they emit light of their own, which may or may not have the same wavelength as the incoming light; the wavelength emitted is more heavily dependent on the composition of the material. For example, planet Mars appears reddish to us since there is a large amount of reddish iron oxide (rust) in its soil. When the white light of the Sun is incident upon the iron and oxygen atoms within the iron oxide materials, they reflect primarily the reddish color light that you see. Remember that sunlight contains all the colors of the spectrum. What happened to the other colors? The blue and green wavelengths of light are mostly absorbed.



Let's put light reflection and absorption in perspective using the optics of Slooh's Online Telescope. The large, metalized light-gathering mirrors in Slooh's reflector telescopes are designed to reflect the light that they gather into the telescope's sensor. However, when the light reaches the telescope's sensor, the sensor absorbs nearly all the incoming light through electron excitation. The results of the excited electrons are ultimately how the images you see are formed. In this specific case, the conversion of light into excited electrons is called photodetection.