$A$ polariser is used to

In which of the following reactions,$H_2O_2$ acts as a reducing reagent?

$A$ line segment $l$ of length $a \ cm$ is rotated about a vertical line $L$ keeping the line $l$ in one of the following three positions: $(I)$ $l$ is parallel to $L$ and is at a distance of $r \ cm$ from $L$,$(II)$ $l$ is perpendicular to $L$ and its mid-point is at a distance $r \ cm$ from $L$,$(III)$ $l$ and $L$ are in the same plane and $l$ is inclined to $L$ at an angle $30^{\circ}$ with its mid-point at a distance $r \ cm$ from $L$. Let $A_1, A_2, A_3$ be the areas so generated. If $r > (a / 2)$,then

$\int_0^{\frac{\pi}{4}} \frac{\cos ^2 x}{\cos ^2 x+4 \sin ^2 x} d x=$

$A$ Young's double slit experiment uses a monochromatic source. The shape of the interference fringes formed on a screen is

For the given figure,if the block remains in an equilibrium position,find the frictional force between the block and the wall in $N$.