AL physics: air wedge

The equation you quoted is a general equation. In the actual sense, optical path difference, instead of geometrical path difference, should be used in interference calculations. This is to cater for the different speeds of light in different media.

For an air wedge, because the medium is air, the refractive index equals to unity. The equation can be reduced to 2d + 入/2


2011-07-23 15:53:33 補充：
The optical thickness of a thin film with physical thickness d and refractive index n is dn. Light reflected from top and bottom of film thus have an optical pathe difference of 2dn.

2011-07-23 15:56:59 補充：
There is genrally a phase change of pi (equivalent to 入/2), either at the top or bottom of the film, for the reflected light when it is reflected from a less dense to a dense medium.

2011-07-23 15:59:05 補充：
Therefore, the total optical path difference = 2dn + 入/2. For constructive interference to occur, 2dn + 入/2 = m入, where m is an integer.