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Homework Statement
Suppose a radiating electric dipole lies along the z axis. Let ##I_1## be the intensity of the radiation at a distance of 10 m and an angle of 90 degrees. Find the intensity (in terms of ##I_1##) at (a) a distance of 30 m and an angle of 90 degrees, (b) a distance of 10 m and an angle of 45 degrees, and (c) a distance of 20 m and an angle of 30 degrees.
Homework Equations
$$Intensity=I=\frac{P_{source}}{Area}$$
The Attempt at a Solution
I've been at a complete loss as to how to solve this. We never covered anything at all similar to this in class, and the textbook has no examples that are similar to this. The solution to this problem states that
$$I(r, \theta)~\alpha ~\frac{sin^2\theta}{r^2}$$
The 'initial conditions' given are 10 m and 90 degrees which is ##I_1##, and the solution then states that
$$I(r,\theta)=I_1~sin^2\theta~(\frac{10m}{r})^2$$
Where are they getting the factor of ##sin^2\theta## from? How does that proportionality lead them to this equation? I was trying to puzzle this out for about an hour last night, and I just spent another half hour staring at it. I feel like I'm missing something really simple right now. Any hints would be very appreciated.