If light interacts with matter without changing its frequency, the process is called elastic scattering because the photons change only their direction and not their energy. The scattered light has the same frequency as the incident light. Rayleigh scattering is one particular elastic scattering process. The key assumption in Rayleigh’s theory is that the scattering particles are small enough compared to the wavelength of the incident light to consider the electric field independent of space within the particles, as was pointed out in Sect. 3.5. In most cases light scattering by molecules can be considered to be Rayleigh scattering.