(Photo modified from http://www.phys.uconn.edu/)
Ray B has to travel twice the thickness of the film as compared with Ray A.
The path difference is 2t.
The refractive index of the film is less than that of glass and so for each reflection, occurring as it does at a rare-dense boundary, there is a 180 degree phase change. The net effect of the phase changes on path difference is therefore zero.
For light of wavelength lf in the film, the condition for destructive interference, at normal incidence, is 2t = 1/2 * lf
Reference:
Duncan, T. (2000). Advanced Physics For Hong Kong. London: John Murray (Publishers) Ltd
Reflected light will experience a 180
degree phase change when it reflects from a medium of higher index of
refraction and no phase change when it reflects from a medium of smaller index.
(http://hyperphysics.phy-astr.gsu.edu)
Relevant information on: http://nikon.com/about/technology/life/others/bluelightcut/index.htm
防藍光鏡片的理論相信只是將 destructive interference 轉變成 constructive interference.
