Imagine the yellow object to be a source emitting photons. The number of photons crossing any spherical shell in a given time is same whatever be its radius and is equal to the number of photons emitted in the same time by the source. If a shell of inner radius 'r1'and outer radius'r2' is imagined, the number of photons entering it at the inner edge and leaving it at the outer edge will be same and the number present in that volume would be number emitted by the Sun in a time equal to the time taken to cross the thickness r2- r1 This applet illustrates these ideas.
Click with left mouse button to draw a circle.
Click and drag to draw an annular ring.
Right click to erase the circle or the ring