Franck hertz experiment problem

[cooper607]

Homework Statement

a beam of electrons bombards a sample of hydrogen. through what potential difference must the electrons have been accelerated if the first line of balmer series is to be emitted

Homework Equations

1/l=R(1/2^2-1/3^2)

E=hc/l

v=E/Q

The Attempt at a Solution

i first tried to find the wavelngth corresponding to first balmer series where i put ni=2 &nf=3.. first let me know if this interpretation is correct

then i found the wavelength l=6.563* E-7 m

according to E=3.03*E-19J

and finally V=E/Q=1.89 V

but the result has to be 12.1 V

please let me know where's the problem

regrds

 

[Doc Al]

The first line of the Balmer series is emitted when the atom goes from n = 3 to n = 2. You found the wavelength and energy of that transition, but that's not what they asked for.

Hint: Before it can drop to a lower energy level it first must reach the higher level.