The problem is to find out the solid angle(adsbygoogle = window.adsbygoogle || []).push({}); ωsubtended by a disc of radiusaat a pointPdistantzfrom its centre along its axis.αis the semi-vertical angle of the disc at the pointPin question.

The answer is supposed to beω= 2π(1 - cosα), according to an online text. However, I find thatω= (Area) / (perpendicular distance)^{2}= (πa^{2})/z^{2}=πtan^{2}α.

I mark my answer in red, in contrast to the "correct" answer in blue.

Any help?

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# Solid angle subtended by disc

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