The wavelength of the photon involved is given by the Rydberg's formula:

#1/lambda = R_H (1/n_1^2 - 1/n_2^2)#,

where #R_H# is the known Rydberg's constant, #R_H = 1.097 times 10^7# #m^{- 1}# and the numbers #n_1# and #n_2# are the initial and final first quantic numbers of energy levels of the electron.

Then we have:

#1/lambda = 1.097 times 10^7# #m^{- 1} cdot (1/2^2 - 1/5^2) = 2.304 times 10^6# #m^{- 1}#.

The radiation frequency can be obtained from the equation of the propagation velocity in the vacuum:

#c = lambda cdot nu# #rArr# #nu = c/lambda = c cdot 1/lambda#

Then:

#nu = 3 times 10^8# #m cdot s^{- 1} cdot 2.304 times 10^6# #m^{- 1} = 6.9 times 10^14# #Hz#.