Paper 2 Quantum Physics Answers

Q1 Photoelectric Effect

1. $E=hc/\lambda =1240/410=3.02\mathrm{eV}$. [2]
2. $K_{max}=hf-\varphi =3.02-2.30=0.72\mathrm{eV}$. [2]
3. $V_s=0.72\mathrm{V}$. [1]
4. Photon energy is unchanged, so $K_{max}$ and stopping potential are unchanged. More photons arrive per second, so photocurrent may increase. [2]

Q2 Line Spectra

1. Photon energy $=-1.50-(-3.40)=1.90\mathrm{eV}$. [2]
2. $\lambda =1240/1.90=653\mathrm{nm}$. [2]
3. Only certain energy levels are allowed, so only certain transition energy differences and photon wavelengths occur. [2]

Q3 X-ray Minimum Wavelength

1. $25.0\mathrm{keV}$. [1]
2. ${\lambda }_{min}=1240/(25000)=0.0496\mathrm{nm}=4.96\times 10^{-11}\mathrm{m}$. [2]
3. It increases intensity/number of X-ray photons but does not change minimum wavelength or characteristic line energies. [2]