First slide

Coherent sources and their importance in interference

Question

A ray of light of intensity I is incident on a parallel glass-slab at a point A as shown in fig. It undergoes partial reflection and refraction. At each reflection 25% of incident energy is reflected. The rays AB and A'B' undergo interference. The ratio $I_{\max} / I_{\min}$ is $B^{`}$

Moderate

A

4 : 1
B

8 : 1
C

7 : 1
D

49 : 1

Solution

From figure $I_{1} = \frac{I}{4} and I_{2} = \frac{9 I}{64} \Rightarrow \frac{I_{2}}{I_{1}} = \frac{9}{16}$

By using
$\frac{I_{max}}{I_{min}} = (\frac{\sqrt{\frac{I_{2}}{I_{1}}} + 1}{\sqrt{\frac{I_{2}}{I_{1}}} - 1}) = (\frac{\sqrt{\frac{9}{16}} + 1}{\sqrt{\frac{9}{16}} - 1}) = \frac{49}{1}$

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