Question

A Light source that radiates E Joules of energy per secondis kept at the centre of a hemisphere. How much energy will the hemisphere receive?

Energy = __

Answer 1

A point source radiates energy equally in all directions. Since it’s a point source the energy will be radiated spherically outwards with the size of the spherical wave-front increasing continuously as the wave propagates.

Now the hemisphere is like a net and the question in one way is asking how much energy it will catch.

The answer is fairly simple, isn't it?

$\frac{E}{2}$.

Hemisphere is half a sphere so it will catch exactly half the energy that the entire sphere will receive.