A point light source of `100W` is placed at a distance x from the centre ofa hole ofradius R in a sheet as shown in figure, Find the power passing through the hole in sheet

Explanation

From figure, the solid angle of cone shown in figure can be given as
`sigma=2pi(1-cos theta)=2pi(1-(x)/(sqrt(R^(2)+x^(2))))`
Power in hole = power given in solid angle `sigma`

`P=(100)/(4pi)xxsigma`
`=(100)/(4pi)xx2pi(1-(x)/(sqrt(R^(2)+x^(2))))`
`=50(1-(x)/(sqrt(R^(2)+x^(2))))"watt"`