A Parallel beam of white light is incident on a thin film of air of uniform thic

Question

A Parallel beam of white light is incident on a thin film of air of uniform thickness. Wavelengths

7200 ˙ A

and

5400 ˙ A

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˙A

B . 4320˙A

C . 5500˙A

D . 6500˙A

Answer: Option B
:
B
The wavelength missing from the reflected spectrum must satisfy the condition,

2 μ t = n λ

where t is
thickness of air film.

2 μ t = n λ_{1} = (n + 1) λ_{2}

n \times (7200) = (n + 1) 5400

∴

n=3
The next wavelength must satisfy the condition

n λ_{1} = (n + 2) λ_{2}

7200 \times 3 = (3 + 2) λ_{2} = 5 λ_{2}

\Rightarrow λ_{2} = 4320 ˙ A

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