Solution

ID: past-exam-of-the-optics-and-electrodynamics-course-of-the-university-of-cambridge/2017/1/c/solution

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 is
The normalization follows from . Equal readings therefore imply
Here , since otherwise all components would vanish. Reversing the axis is immaterial, so choose . The complete family is
Define , , and . Then , and the absolute intensity fixes
For 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.

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