To understand atomic absorption it is necessary to take a look at some of the concepts involving the nature of light itself.
Light is just a form of electromagnetic radiation. The term electromagnetic radiation refers to the propagation of energy through a medium (or a vacuum) in the form of oscillations of an electrical nature with a corresponding perpendicular magnetic field (motion of electrical fields causes a magnetic field and vice versa)
So electromagnetic radiation is a form of energy that passes from one place to another at the speed of light in the form of waves.
Waves have various characteristics, such as velocity (the speed of light), wavelength (the distance from a point on one wave to the equivalent point on the next wave), frequency and amplitude.
The amplitude is proportional to the intensity of light and not particularly important at the moment. The frequency is the number of wavelengths that pass a specific point per second. There is a very simple relationship between the velocity of a wave, the wavelength and the frequency.
c (the velocity) = lambda (the wavelength) x f (the frequency)
c is a constant (the speed of light)
The energy of a wave is difectly proportional to its frequency and hence, inversely proportional to its wavelength. Both are connected to the energy be the Planck constant ‘h’.
Energy = h x f
Energy = hc/wavelength
In atomic absorption events, the energy of the incoming wave must have the exact energy that is needed for an electron to be promoted to a higher level. It absorbs this energy and gets promoted. Detection systems see this specific wavelength (or frequency) removed from the spectrum.
An absorption spectrum thus appears as a series of dark lines superimposed on a continuous spectral background. This process need not necessarily take place in the visible region of the spectrum (which is tiny compared to all the available electromagnetic spectrum, it occupies the wavelengths from about 400 -700 nm) in which case the detection cannot be visible, it must rely on apparatus designed to detect absorption in specific regions of the electromagnetic spectrum.