State what the term photon means.
Light in a beam has a continuous spectrum within the visible region. The photons in the light have energies from $1.60\,\text{eV}$ to $2.60\,\text{eV}$. The beam passes through some hydrogen gas. It then passes through a diffraction grating, and an absorption spectrum is observed. All of the light absorbed by the hydrogen is re-emitted. Explain why dark lines are still observed in the absorption spectrum.
Some of the electron energy levels in a hydrogen atom are shown in Fig. 10.1. The dark lines in the absorption spectrum result from electron transitions between energy levels. On Fig. 10.1, draw arrows to indicate the initial electron transitions between energy levels that could produce dark lines in the absorption spectrum.
Calculate the minimum wavelength of the light in the beam.