When a large bubble rises from bottom of a water lake to its surface, then its radius doubles. If the atmospheric pressure is equal to the pressure of height $H$ of a certain water column, then the depth of lake will be

A block of mass $M$ is pulled along a smooth horizontal surface with a rope of mass $m$ by force $F$. The acceleration of the block will be

In the network shown cell E has internal resistance $r$ and the galvanometer shows zero deflection. If the cell is replaced by a new cell of emf $2 E$ and internal resistance $3 r$ keeping everything else identical, then

In hydrogen emission spectrum, for any series, the principal quantum number is $n$. Corresponding maximum wavelength $\lambda$ is ( $R=$ Rydberg's constant)