Question

A given (point) source, of power P, radiates light, which has a peak value, B₀ , for its magnetic field, at a point distant r from the source. The graph, showing the dependence of B₀ on r, would, then, be the graph labelled as graph

(1) K

(2) L

(3) M

(4) N

Answer 1

(2) L

The intensity, I, of the light from a (point) source of power P, at a point distant r from it, is

I = P/4πr²

The intensity I also equals c times the total energy density of the relevant em wave. This implies that

This dependance of B₀ on r, is shown in the graph labelled as graph L.