The energy of an electron in the $3^{rd}$ orbit of $H$-atom (in $J$) is approximately:

Which of the following statements are correct,if the threshold frequency of caesium is $5.16 \times 10^{14} \ Hz$?
$A.$ When $Cs$ is placed inside a vacuum chamber with an ammeter connected to it and yellow light is focused on $Cs$,the ammeter shows the presence of current.
$B.$ When the brightness of the yellow light is dimmed,the value of the current in the ammeter is reduced.
$C.$ When a red light is used instead of the yellow light,the current produced is higher with respect to the yellow light.
$D.$ When a blue light is used,the ammeter shows the formation of current.
$E.$ When a white light is used,the ammeter shows the formation of current.
Choose the correct answer from the options given below:

What is the number of waves formed by an electron in the $5^{th}$ Bohr orbit of a hydrogen atom?

When atoms are bombarded with alpha particles,only a few in a million suffer deflection,while others pass through undeflected. This is because:

What is the ratio of the wave number of the first line (lowest energy line) of the Balmer series of the $H$ atomic spectrum to the first line of its Brackett series?

The ratio of the area enclosed by the second orbit to the first orbit is ......