Question: Two rectangular pieces of plane glass are laid one upon the other on a table. A thin strip of paper is placed between them at one edge, so that a very thin wedge of air is formed. The plates are illuminated at normal incidence by 544nm light from a mercury-vapor lamp. Interference fringes are formed, with 13.0 fringes/centimeter.
1- Find the angle of the wedge.
If possible just a brief explanation how you got the answer?