25. 0/2 points | Previous Answers My Notes Two rectangular, optically flat, glas

Question

25. 0/2 points | Previous Answers My Notes 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 500 nm is incident perpendicularly onto the top plate. The air between the plates acts as a thin film. Nineteen dark fringes and eighteen bright fringes are observed across the top plate. If the distance between the two plates along the separated edges is increased by 2500 nm, how many dark fringes will there then be across the top plate? x dark fringes Incident light

Explanation / Answer

you have 2t=m where t is the thickness of the paper, m is the number of bands and is wavelength

solve for t

t=m/2

convert into same units

500nm=0.0000500cm

t = 19*0.0000500cm /2 = 4.75*10-4 cm

Now thickness is increased by 2500 nm i.e. 2.5 *10-6 cm

so total thickness T = 4.75*10-4 m + 2.5 *10-6 cm = 4.775 *10-4 cm

Now calculate for m,

m = 2T/ = 2*4.775 *10-4 cm / 0.0000500cm = 19.1

i.e. approximately 19

So, because seperation is very small, the number of dark bands remains same

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