There appears to be a lot of material on the internet claiming that viruses can be only seen with electron microscopes, and not with light microscopes. To the contrary, for example this old paper published in Nature states that viruses can be seen using phase-contrast (light) microscopy. Who's correct? What is the smallest thing I can see using a phase-contrast microscope?
It is true for most viruses. They have a size of roughly 1/100 of bacteria (or smaller), so they are too small to be seen in light microscopy. According to Wikipedia the maximum limit with light microscopy is around 1500x magnification (or making structures, which are at least around 200nm in size visible).
A lot of viruses are smaller, for example the influenza A virus is around 80-120nm, the HIV virus around 120nm and the rhinovirus which causes the common cold around 30nm. They would only be visible in electon microscopy. The measles virus on the other hand has between 350 and 400nm and should be visible in light microscopy. However, since you are very close to the region where you reach the diffraction limit for visible light, it is usually not used very much.
It is true generally but it's not an absolutely truth.
There is a group of giant viruses which can be seen under the traditional light microscope - such as the pandoravirous - which its size is about 1000 nano-meters.
Viruses are usually too small to be seen at the light microscope level. However: large virus conglomerates within cells can be seen under a light microscope. In that case they're called "inclusion bodies". These have diagnostic value. For example in the case of rabies, were those so-called "Negri-bodies" provide the beyond-any-doubt post mortem diagnosis of rabies.