Question

In the franck hertz experiment, why does the light appear after the grid and not before?

Answer 1

Accelerating charges emit radiation. I'm not entirely positive here, but I would guess that they only emit the visible spectrum after the grid since they are decelerating and thus losing the proper amount of energy on the opposite side to give off visible light.

Answer 2

But the whole point of the experiment is to show that mercury is the giving of discreet amounts of energy, The eneregy lost when the excited mercury atom goes back to its ground states is what is supposed to give the light. But for some reason it only gives of light after the grid.