Two rectangular, optically flat, glass plates (n = 1.60) are in contact along on

Question

Two rectangular, optically flat, glass plates (n = 1.60) are in contact along one edge and are separated along the opposite edge (figure below). Light with a wavelength of 700 nm is incident perpendicularly onto the top plate. The air between the plates acts as a thin film. Fifteen dark fringes and fourteen bright fringes are observed across the top plate. If the distance between the two plates along the separated edges is increased by 2800 nm, how many dark fringes will there then be across the top plate?

Explanation / Answer

2t=ml

m=15

-->t=ml/2=15*700e-9/2=5.25e-6 m=5250 nm

t1=5250+2800=8050 nm

m1=2*t/l=2*8050e-9/700e-9=23 dark fringes

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