Beer

1. The classical law and its history

When electromagnetic radiation, such as ultraviolet or visible light, passes through a transparent medium that contains an absorber of that illumination, the radiation’s intensity diminishes steadily with passage through the medium. Commonly the radiation is in the form of a collimated beam that impinges perpendicularly on a slab of width L of the medium, as suggested diagrammatically in Figure 1.One may conjecture that, at any illuminated plane x within the medium, the decrease in the intensity I of the radiation with distance would be proportional to the uniform concentration c of the absorber and to the local intensity of the light at that point; that is

[math] \frac{\mathrm d}{\mathrm d x} ( I(x) )=-\alpha cI(x) (1) [/math]