The first microscopes used light and glass lenses. A light microscope can magnify up to about ×1500 and shows cells and larger structures such as the nucleus, cytoplasm, cell wall and chloroplasts. Its limit is its resolution, which is the ability to tell two points that are close together as separate. Structures that are smaller than this limit just blur into one.
In the 1930s the electron microscope was developed. It uses a beam of electrons instead of light. This gives it a much higher resolution and a much greater magnification, so very small structures can be seen in far more detail. The specimens must be dead and kept in a vacuum, but the images are much clearer.
Ribosomes and plasmids are too small to see with a light microscope but can be seen with an electron microscope. Electron micrographs show the folded inner membrane of a mitochondrion, which gives a large surface area for aerobic respiration, and the internal membranes of chloroplasts. Seeing structures this clearly helped scientists work out what each one does, so our understanding of sub-cellular structures has increased.