Take the 2-minute tour ×  Refraction: light changes direction of
propagation when entering a material with a different refractive
index.  enter image description here  Does the direction of
propagation of light change sharply and almost instantaneously
(as shown in the diagram) or smoothly?  share|improve this
question  2 Answers 2  up vote 2 down vote accepted  I don't
understand the down vote on the OP. In order for Snell's Law
($n_1 \sin{\theta_1} = n_2\sin{\theta_2}$) to be valid, certain
conditions have to be met. Usually we assume the conditions are
met, and draw the simple diagrams such as yours. These represent
idealizations of real situations. For example, the ideal
interface is discrete, changes abruptly, and is perfectly flat,
and the materials are homogenous. At a microscopic level, all
real interfaces consist of meetings of molecules, so are not
discrete, abrupt, nor flat, and materials are not homogeneous
(they are made of atoms and molecules).  However, from a
macroscopic perspective, if we don