A sample tube consisted of atomic hydrogen in their ground state. A student illuminated the atoms with monochromatic light, that is, light of a single wavelength. If only two separate emission lines in the visible region are observed, what is the wavelength (or wavelengths) of the incident radiation?

The wavelength is

Explanation:

Generally the series whose emission  line show on the visible spectrum is the

Balmar series so this two emission line seen on the visible spectrum  could either be due to the move of electron from

OR

This implies that the first excitement is from

So the energy change due to the excitement is mathematically represented as

substituting values

This energy change can also be represented as

So  

=>    

Where   is the Rydberg constant with a value of  

              h is the Planck's constant with values  

                c is the speed of light with value  

So