Question: Thomson says that the photographic plate that took the image shown in Fig. was 32.5cm from the gold foil. Use that number and the fact that the diameter of the (200) diffraction ring was 2.5cm for 24.6keV electrons to find the lattice constant of the gold foil. How does your result compare with the value given in Table? Warning: Remember that the angle of the diffraction ring relative to the incident beam is twice the diffraction angle θ.
Fig. - The diffraction pattern from electrons on a polycrystalline gold foil observedby G.P. Thomson