Cell Biology/Introduction/Cells Under Microscopes
1 μm = 10-6m = 10-3mm
1 nm = 10-9m = 10-3μm
One can use a light microscope to see living cells, its plasma membrane, nucleus and probably some most prominent organelles (with the help of a computer), as it can not see things smaller than 0.2 μm (due to light wavelength, not to the quality of lens).
One can see things about a few nm in size. The organelles appear very clearly. Although you have to kill all cells before you see them. You cannot see wet cells under an electron microscope as well. You have to cover your specimen with a very thin layer of heavy metal before you can work with it.
Some very large cells can be seen with the eye alone. An example of this is gromia sphaerica which has a maximum specimen size of 38 mm (millimeters).