Dealing with Reflections and Refractions.

Each plate reflects 25% and transmits(refracts) 75% .

Intensity of incident ray (Initial intensity I ) after each reflection and refraction is shown in figure.

Interference pattern will take place in between rays 1 and 2 .

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

$\therefore \quad \quad \quad \quad \frac { { I }_{ max } }{ { I }_{ min } } ={ \left( \frac { \sqrt { { I }_{ 1 } } +\sqrt { { I }_{ 2 } } }{ \sqrt { { I }_{ 1 } } -\sqrt { { I }_{ 2 } } } \right) }^{ 2 }=\frac { 49 }{ 1 } =\quad \boxed{49}$