Explain how the emission spectrum gives evidence for the existence of discrete energy levels for the electron in a hydrogen atom.
Fig. 9.2 presents five of the energy levels in the hydrogen atom. The radiation wavelengths shown in Fig. 9.1 correspond to transitions to the $-3.400\,\text{eV}$ level in Fig. 9.2. Show that the energy level $X$ is $-1.51\,\text{eV}$.
State the name of the effect responsible for the change in wavelength.
State what this effect shows about the motion of the galaxy.
Use one of the lines in Fig. 9.1 together with the matching line in Fig. 9.3 to find the speed of the distant galaxy relative to the observer.
The galaxy in (b) is known to be a distance of $5.7 \times 10^{24}\,\text{m}$ from the Earth. Use your answer to part (b)(iii) to find a value for the Hubble constant $H_0$.