Resolution (or strictly resolving power) is defined as the closest spacing of two points which can be resolved by the microscope to be separate entities. This simulation shows two sets of Airy rings showing the variation in light intensity across the rings. The Rayleigh criterion states that when the maximum intensity of one Airy disc coincides with the minimum of the second, the two points can just be distinguished

Try changing the separation of the two discs. Look at the simulated image at top left and see when you can clearly distinguish two separate points from one elongated one. Move the slider using the mouse and then wait - slower computers may take a few seconds to recalculate the plots.

What would the image of a small point look like if we had square apertures in the TEM rather than the usual circular ones?