The energy of a hydrogen atom in its ground state is $-13.6 \,eV$. The energy of the level corresponding to $n=5$ is ........ $eV$.

$A$ person is observing a bacteria through a compound microscope. For better observation and to improve its resolving power,he should:

The area (in square units) of the circle which touches the lines $4x + 3y = 15$ and $4x + 3y = 5$ is

Two identical bodies have temperatures $277^{\circ} C$ and $67^{\circ} C$. If the surroundings temperature is $27^{\circ} C$, the ratio of loss of heats of the two bodies during the same interval of time is (approximately) (in $19 : 1$)

Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is

Find the binomial probability distribution whose mean is $3$ and variance is $2$.