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Q.

Blue light of wavelength 480 nm is most strongly reflected off a thin film of oil on a glass slide when viewed near normal incidence Assuming R.I. of oil is 1.2 and that of glass is 1.6 find  minimum thickness of the oil film (other than zero).

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a

150 nm

b

240 nm

c

200 nm

d

480 nm

answer is C.

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Detailed Solution

Oil is Optically denser than air, also glass is optically denser than oil. So the incident rays at top and bottom surfaces of oil film, both suffer an additional phase difference of 180o upon reflection.

Therefore, for the formation of bright fringe

2\,\mu t\,\,cos\,r=n\,\lambda,n=0,1,2,......

For near normal incidence,

r \simeq {0^o}\
\therefore t=\frac{n\lambda}{2\mu}
\therefore t_{min}=\frac{1\times480}{2\times1.2}\,nm=200 \,nm.
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Blue light of wavelength 480 nm is most strongly reflected off a thin film of oil on a glass slide when viewed near normal incidence Assuming R.I. of oil is 1.2 and that of glass is 1.6 find  minimum thickness of the oil film (other than zero).