White light is incident on a cloud of cool hydrogen gas, as illustrated in Figure I below.
I
The spectrum of the light emerging from the gas cloud is found to contain a number of dark lines.
Explain why these dark lines occur.
Some electron energy levels in a hydrogen atom are illustrated in Figure II below.
II
One dark line is observed at a wavelength of 
.
Calculate the energy, in 
, of a photon of light of wavelength .
On figure II above, draw an arrow to indicate the energy change that gives rise to this dark line.
.
