Viruses that are invisible to the naked eye can be seen using an electron microscope. Viruses vary relatively widely in size, with the largest viruses being about 300 nm, such as poxviruses, and the smallest viruses being about 20 nm, such as fine DNA viruses. Since viruses are so small that they cannot be seen with the naked eye or even with an ordinary light microscope, they can only be seen using an electron microscope. Electron microscopes use an electron beam to illuminate the specimen, and the wavelength of the electrons is very long, making the resolution of the electron microscope significantly higher than that of an optical microscope. A typical transmission electron microscope has a resolution of about 0.2um and is used to see the morphology and structure of viruses.