Photo cameras rely on photosensitive areas that react to light. To understand the physical principles digital cameras are based on, it is also neccessary to understand the nature of light.
The light model of electromagnetic waves: In general, electromagnetic waves are commonly known from radio waves, micro waves, x-ray or nuclear radiation (gamma rays). Visible light is only a small part of the entire electromagnetic spectrum. Due to the electromagnetic character of light, it also propagates in waves. The main difference between all types of radiation is their energy which is determined by their individual wavelengths. In the visible light spectrum, the wavelenght of a light ray also determines it’s specific color. The chart below shows the relation between the visible light spectrum and the entire electromagnetic spectrum.
As shown in the electromagnetic spectrum chart, the visible light ranges from around 400nm to 700nm. Here is a more precise allocation of colors to individual wavelenghts:
|700 to 630 nm||red|
|630 to 590 nm||orange|
|590 to 560 nm||yellow|
|560 to 490 nm||green|
|490 to 450 nm||blue|
|450 to 400 nm||purple|
The longer the wavelength of visible light, the more red the color. Likewise the shorter wavelengths are towards the violet side of the spectrum. Wavelengths longer than red are referred to as infrared (IR), while those shorter than violet are ultraviolet (UV). It is important to mention that white light is not related to an individual wavelength but is composed of the simultaneous presence of all wavelenghts.
However, there is a fact that the energy of light cannot be delivered in a completely linear way, but only quantized. This fact lead to the assumption that light must be more than just a wave, but also has properties of tiny energy particles.
The photon model of light: As proposed by Einstein, light consists of photons, extremely small quantums of energy. The reason that photons are able to travel at the speed of light is due to the fact that they have no mass and therefore, Einstein’s famous equation E=mc² cannot be used. Another formula – devised by Planck – is used to describe the relation between the energy of a photon and it’s frequency: E = hf or E = hc / λ (h is the Planck constant and is a fixed value).