White light may be considered to be mixture of waves of `lambda` ranging between `3900 Å` and `7800 Å`. An oil film of thickness `10,000 Å` is examined normally by the reflected light. If `mu=1.4`, then the film appears bright for

Options

A. (a) `4308Å, 5091 Å, 6222 Å`
B. (b) `4000 Å, 5091 Å, 5600 Å`
C. (c) `4667Å, 6222 Å, 7000Å`
D. (d) `4000Å, 4667Å, 5600Å, 7000Å`

Answer

A

Explanation

The film appears bright when the path difference `(2mutcosr)` is equal to odd multiple of `lambda/2`.
i.e., `2mutcosr=(2n-1)lambda//2` where `n=1, 2, 3….`
`:. lambda=(4mutcosr)/((2n-1))`
`=(4xx1.4xx10,000xx10^(-10)xxcos0)/((2n-1))=(56000)/((2n-1))Å`
`implieslambda=56000Å, 18666Å, 8000Å, 6222Å, 5091Å, 4308Å, 3733Å`.
The wavelength which are not within specified range are to be refracted.