make a new file, go to the gradient tool and select the radial gradient tool. press [edit...] at the radial gradient options box. you should get to the gradient editor. move the sliders, so that the backgroundcolor is at 100% and the foregroundcolor is at 0%. with the little thing that is marked red in the picture, you can alter how much highlight the gradient will have - you will see how it works later.

make a radial selection. a good idea is to use the guides to define the middle of the circle and make the circle with pressed (shift) + (alt), so that it starts at the middle and is circular instead of elliptical. use the radial gradient tool from the near-middle of the selection to the right bottom. when you play around with it, you will quickly realize how the whole thing works.

drop the current selection, and make another (concentric) circular selection on the same layer. if you have used the guides, and have "snap to guides" activated, there should be no problem to make it perfectlic concentric. invert it (strg)+(i)

make another smaller concentric circle, and invert it again.

this is the whole trick.