If you looked down the most powerful light microscope in the world, you’d be able to distinguish individual objects that are around 200 nanometres apart – roughly 1/500 the width of a human hair. But objects closer together than that would just merge into one. This is because the wavelength of visible light is longer than 200 nanometres.
But electron microscopes use beams of electrons, instead of light. The wavelength of these electron beams is much shorter, allowing scientists to see structures as small as 1 nanometre (1 millionth of a millimetre).