A parallel beam of white light is incident on a thin film of air of uniform thickness. Wavlength `7200 Å` and `5400 Å` are observed to be missing from the spectrum of reflected light viewed normally. The other wavelength in the visible region missing in the reflected spectrum is

Options

A. `6000 Å`
B. `4320 Å`
C. `5500 Å`
D. `6500 Å`

Answer

b

Explanation

The wavelength missing from the reflected spectrum must satisfy the condition,`2 mu t = n lambda`, where t is thickness of air film.
`2 mu t = n lambda_(1) = (n + 1) lambda_(2)`
or `n xx (7200) = (n + 1) 5400`
`:.n = 3`
The next wavelengths must satisfy the condidtion,
`n lambda_(1) = (n + 2) lambda_(2)`
or `7200 xx 3 = (3 + 2) lambda_(2) = 5 lambda_(2)`
`implies lambda_(2) = 4320 Å`.