Past exam of the optics and electrodynamics course of the University of Cambridge 2017 1 c Solution 2026-10-18
Use axes adapted to the two orthogonal rotations in the diagram:For a linearly oscillating Hertzian dipole with real unit axis and total time-averaged radiated power , the angular distribution of electric dipole radiation isThe normalization follows from . Equal readings therefore implyHere , since otherwise all components would vanish. Reversing the axis is immaterial, so choose . The complete family isDefine , , and . Then , and the absolute intensity fixesFor the measured distance and intensity, the allowed axes and powers are:
- : , .
- or : , for either axis.
- : , .
The intensity data do not select a unique orientation or total power. This is the dipole-axis ambiguity from three intensity measurements: the readings constrain squared projections and lose their signs. For example, the first branch makes an angle with , but the other branches are equally consistent with the three measurements. A further suitable observation direction or polarization measurement is needed to remove the ambiguity. The calculation uses radiation-zone intensities, as appropriate to the Hertzian dipole model.