Why do we say “approximation” in the dipole approximation in spectroscopy?

In the dipole approximation, the following relation holds:

$$\hat{V} = -\hat{\mu} \cdot \vec{E}.$$

When we say "approximation", I guess we want to point out that we are dealing with linear spectroscopy, where we have the polarizability of the molecule proportional to the electric susceptibility of a molecule:

$$\vec{p}(\omega) = k\chi \vec{E}.$$

If the wavelength of the light is not larger than the atom size, the dipole approximation is no longer a good approximation. This is probably also true for large molecules. I'm not sure if this has to do with large molecules no longer having symmetry, so transitions might be allowed which otherwise should be forbidden.

Is this correct?

• So for non-linear spectroscopy the dipole approximation $\hat{V}=-\hat{\mu}\overrightarrow{E}$ is no longer adequate? – laminin Jun 26 '15 at 15:12