A rectangular block of glass is placed on a printed page lying on a horizontal surface. The minimum value of the refractive index of glass for which the letters on the page are not visible from any of the vertical faces of block is 

For object not to be visible from vertical face (AC) any ray starting from object should not come 
out of vertical face. i.e., it should suffer TIR at vertical face.
Assume BAC to be prism of angle  $A={90}^0$. The condition for no ray to emerge from second 
refracting surface of prism is  $A\ge 2C$
$\Rightarrow 90\ge 2C$
Or,   $C\le {45}^0$
$\Rightarrow \sin C\le \left(1/\sqrt{2}\right)$

Or, $n\ge \sqrt{2}$