Describe how interference fringes are formed in wedge shaped film of air.
Describe how interference fringes are formed in wedge shaped film of air.
solution
A monochromatic light incident normally at O is partially reflected and partially transmitted on the at the bottom of the top glass slide.
The transmitted light is reflected at B on the top of the lower glass slide.
The two reflected wave trains are coherent and when they superpose above the upper side of top glass slide, they produce interference pattern in form of alternative dark and bright fringes parallel to the line of intersection of the slide.
The path difference is 2t where t is the small thickness of the air film between the glass slides at O.
At C where the path different is apparently zero, a bright fringe is expected, however, a dark fringe is observed. This is due to phase change of π radians (equivalent to path difference of λ/2 ) that occurs when a wave is reflected at a denser medium.
Hence for constructive interference (bright fringes) (2t + λ/2) , for n = 1, 2, 3, …
Destructive interference (dark fringes) (2t = nλ) , for n = 1, 2, 3, …
