3.1.1 Electromagnetic Waves and Photons
Light is the common name of electromagnetic radiation (EMR) that we can see. Lamps, lasers, and light emitting diodes (LEDs) that generate light can also emit EMR, which is not visible. However, they have specific features characteristic to the visible light, such as photochemical action for the shorter wavelengths (violet color) and thermal action for the longer ones (red color). Thus, two neighboring regions of EMR (i. e., ultraviolet (UV) and infrared (IR)) also belong to light. UV and IR light can be seen (visualized) with the help of matrix photodetectors such as CCD or IR thermal cameras. Electromagnetic radiation propagates in vacuum or different media in the form of electromagnetic waves, which are periodical oscillations of electrical and magnetic fields in time and space.
Light can be also described as a stream of photons. Photon is a quantum of EMR, usually considered as an elementary particle that has energy Eph = hvor h(c/X), where h is the Planck’s constant (a physical constant that is used to describe the sizes of quanta; it plays a central role in the theory of quantum mechanics, and is named after Max Planck, one of the founders of quantum theory), v is the frequency of light, c is the speed of light, and l is the wavelength of light. Wavelength is the distance between two adjacent peaks in electric or magnetic fields of EMR of a light wave, measured typically in nanometers (nm) or micrometers (pm): 1pm is 10-9 meter (m) and 1 pm is 10-6 m; h = 6.626 x 10-34 joules x seconds; c = 3 x 108 m/s in a free space, vis expressed in hertz (Hz), 1 Hz is s-1, because of high frequency of light its frequency typically expressed in THz: 1 THz is 1012 Hz. For example, IR radiation with the wavelength of 10 pm is oscillating with the frequency of 30 THz.
To evaluate the total energy of the light beam, we need to account for each photon that was detected or interacted with the target; if the total number of photons is N, then Etotal = N x Eph = N x hv.