Whenever a wave encounters an obstacle a number of processes occur. For large objects we envisage reflection and transmission with refraction and, in ·many cases, absorption. These phenomena can be described with the aid of ray tracing or geometrical optics, but they do not completely describe the interaction. Diffraction also occurs, and this can only be described by the properties of waves, wave optics. When the object is less than or of the order of the wavelength these processes cannot be so simply understood. The whole interaction is governed by wave optics, and the interactions are lumped together under the heading 'scattering'. Associated with the above there may be changes in frequency of the wave. This may arise due to the Doppler effect if the obstacle is moving or changing in time in any way. Also there can be changes in the energy of the object which must be matched by the wave, such as, for example, in the Raman effect.