Question

the wavelength of the third side of the Balmer series of a hydrogen atom

Answer 1

Balmer series. The Balmer series is the name given to a series of spectral emission lines of the hydrogen atom that result from electron transitions from higher levels down to the energy level with principal quantum number 2.
Four of the Balmer lines are in the technically "visible" part of the spectrum, with wavelengths longer than 400 nm and shorter than 700 nm. Parts of the Balmer series can be seen in the solar spectrum. H-alpha is an important line used in astronomy to detect the presence of hydrogen.

Answer 2

for xth line of a series : n2 = n1 + x
here n2 = 2+3=5
1/w = Rh (1)^2 (1/4 - 1/25)
put Rh and get the answer..