A beam of light consists of a continuous range of wavelengths from 
to 
. The light passes through a cloud of cool gas, as shown in Figure I below.
I
The spectrum of the light emerging from the cloud of cool gas is viewed using a diffraction grating. Explain why this spectrum contains a number of dark lines.
Some of the electron energy levels of the atoms in the cloud of gas are represented in Figure II below.
II (not to scale)
Light of wavelength has a photon energy of 
. Calculate the photon energy, in 
, of light of wavelength .
Use data from (i) and your answer in (i) to show, on figure II, the changes in energy levels giving rise to the dark lines in (a).
.
