A thin film of soap solution `(mu_(s)=1.4)` lies on the top of a glass plate `(mu_(g)=1.5)`. When visible light is incident almost normal to the plate, two adjacent reflection maxima are observed at two wavelengths 420 and 630 nm. The minimum thickness of the soap solution is

Options

A. 420 nm
B. 450 nm
C. 630 nm
D. 1260 nm

Answer

B

Explanation

Condition of maximum intensity,
`2mut= nlambda`
For n and (n-1) maxima
`nlambda_(1)= (n-1)lambda_(2)`
`n xx 420 = (n-1)630`
`implies n= 3`
Maximum order where they coincide
`2 xx 1.4 xx t= 3 xx 420implies t= (3 xx 420)/(2 xx 1.4)= 450 nm`