First slide

Coherent sources and their importance in interference

Question

A light of wavelength 5890 Å falls normally on a thin air film. The minimum thickness of the film such that the film appears dark in reflected light

Moderate

A

$2 .945 \times 10^{- 7} m$
B

$3 .945 \times 10^{- 7} m$
C

$4 .95 \times 10^{- 7} m$
D

$1 .945 \times 10^{- 7} m$

Solution

If thin film appears dark
$2 μtcos r = nλ for normal incidence r = 0^{\circ} \Rightarrow 2 μt = nλ \Rightarrow t = \frac{nλ}{2 μ} \Rightarrow t_{min} = \frac{λ}{2 μ} = \frac{5890 \times 10^{- 10}}{2 \times 1} = 2 .945 \times 10^{- 7} m$

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