It is ordinarily quadratic in the electric field, although it  can be linear when there is a permanent electric dipole moment.  
It is ordinarily quadratic in the electric field, although it  can be linear when there is a permanent electric dipole moment.  First observed by the German scientist for whom it is named as a  splitting of the Balmer lines in hydrogen, it is the electric  analogue of the Zeeman effect. FTP, what is this effect, which  earned its discoverer, later a prominent Nazi, the Nobel Prize for  Physics in 1919?
It is ordinarily quadratic in the electric field, although it  can be linear when there is a permanent electric dipole moment.  
{Stark} effect