Why do street lights and head lights of vehicles look like stars when viewed through glass?

   The star-like appearance can be noticed only when a considerable amount of light reaches the glass (or any other transparent sheet). The phenomenon deals with two rays of light: the ordinary ray and the extraordinary ray. The ordinary ray obeys the law of light and gets diffracted. But the extraordinary ray does not. That’s why when we see light sources through a sheet of glass, the centre is bright.

As the ordinary ray falls on the outer surface of the glass, it gets diffracted and bends at an angle. It is further bent as it escapes from the inner side of the glass plate (towards our eye). This deviation in the path of the ray results in the characteristic star shaped appearance. Also, the diffracted light rays undergo interference and superposition. These phenomena take place only when the rays emerging are in phase with one another. The rays which do not have the same amplitude or wavelength do not superpose. Thus we get a glittering appearance resembling a star.