防反光鏡片 (防藍光鏡片的相反理論)

(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 l_f in the film, the condition for destructive interference, at normal incidence, is 2t = 1/2 * l_f

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.