The majority of FITS files contain images taken by ground-based telescopes in the visible light. To understand the nature of these images, the developer implementing image processing applications for the astronomy should understand the nature of the light. This tech note introduces the basic formulas needed to understand the nature of the visible light. Throughout this portal they are used to explain how different sensors work, and the many aspects of noise reduction algorithms.
Note: We will provide additional tech notes for images in non-visible spectra (e.g. infrared, radio, or high energy particles) later.
Light is electromagnetic radiation. Moving charges generate electromagnetic fields that propagate away from their sources. Only the relationship between frequency and wavelength (see below) distinguish between different types of electromagnetic waves that astronomers observe: radio, infrared, visible, x-rays, and gamma-rays. As a physical phenomenon, visible light is not something special. It is special to us — humans, due to the fact that it is the only part of the spectra we can "see". One reason evolution made us see "visible" light is because the Earth atmosphere is particularly transparent for visible light and the wavelengths within the visible spectrum and is well suited for us to see fine details (like the small letters of this tech note).
The relationship between the wavelength and the frequency for electromagnetic radiation is:
λν = c (Equ. tn_0001.1)
where λ as the wavelength, ν the frequency, and c the speed of light. For example, the yellow-green light (to which human eye is most sensitive) is 550 nanometers (length) and 545 THz (frequency).
Classical physics (before the turn of the 20th century) treated light as a waves, which was however revised as it was discovered that energy carried by light is quantised, behaving like a stream of particles, each carrying specific energy. The particle behaviour of light is especially important for the electronic sensors used in modern astronomy.
More precisely, the energy carried by a single photon (light particle) of wavelength λ is:
E = hc / λ [eV] (Equ.tn_0001.2)
where c is again the speed of light, and h is Plank's constant. The product of hc is 1240 eV nm (electron-volt nanometers). That way each photon of yellow-green light carries an energy potential of ca. 2.25 eV. Long-wavelength photons are less energetic and short-wavelength photons are more energetic.
When examining the interaction of light with detectors (i.e. CCDs), where the quantised nature of light is evident, it is more practical to use its particle nature. Why this is important will be described in a technical note discussing the architecture of a CCD.
When discussing how light forms images, travels through space, bounces off mirrors, and bends as it passes through transparent materials (e.g. glasses) it is more practical to use the ray and wave description of light.