Why viruses cannot be seen? With the coronavirus pandemic, a lot of websites are publishing articles about viruses. In particular, I've seen some of these stating that viruses cannot be seen because they are so tiny they cannot reflect light. This seems to be true at first sight but I've been intrigued ever since I put some real thought about it. Early quantum physics teaches us that electrons can absorve and re-emit photons. Thus, if we picture a light beam hitting a single atom, the electrons of this atom might absorb some photons and change their state. If these electrons readily re-emit these photons, the light is been reflected and, otherwise, the light is absorbed. This very simple model shows that even electrons, which are pretty smaller than a virus, can emit photons and 'reflect' light. Of course, we cannot see electrons either but this could be explained by the fact that a single eletron emits a little number of photons per second and our eyes cannot process an image. But, if we put a huge number of atoms together, the net effect allows us to see these objects, their colour and so on. So, is the size of a virus the real explanation to the fact that they cannot be seen or should the correct explanation be due the fact that we cannot put together as many viruses as it would take in order to have an proper image of something?
PS: I know the interaction between light and matter are better explained using QED but I'm proposing a toy model here just to thing things classicaly, if possible.
 A: Per the question, "So, is the size of a virus the real explanation to the fact that they cannot be seen should the correct explanation be due the fact that we cannot put together as many viruses as it would take in order to have an proper image of something?^".
In fact if you had a teaspoonful of viruses you would be able to see them easily.
The reason you cannot see a single virus by eye is, as you surmise, simply because viruses are too small to scatter enough photons for your eye to perceive unless illuminated with a very, very bright light source
There are types of optical microscopy in which light emitted by a single molecule is the only light that is observed.  A Nobel prize was awarded for that work.
A: The human eye is an optical instrument, which is subject to diffractive limits, optical aberrations and biological limitations (receptor size in the retina). 
The angular resolution of the human eye is approximately 1 minute of an arc (1/60th of a degree).
The diameter of the smallest object that can be resolved by the unaided eye at $25~\rm cm$ (shortest object distance for clear vision) is $75~\rm\mu m.$ 
Since most of the virus have sizes in the nanometer range, it is not resolvable for the human eye. 
You can aid your eyes by using microscopes, which magnifies the virus into large images which can be resolved by your eyes. Note that just like your eyes, microscopes have limitations too. The resolution of a microscope can be increased by using lights of shorter wavelengths. For imaging extremely small objects like viruses, you need an electron microscope which uses electrons instead of light. 
A: Seeing means imaging using optical methods, which range from 400 nm (violet) to 750 nm (dark red). A single SARS-CoV-2 virus measures 120 nm so the virus so is much smaller than optical wavelengths. The images of virions that we do have were made by scanning electron microscopes, which has a resolution limit of better than 1 nm. Atoms measure about 0.1 nm and can be visualized by atomic force microscopy. Images of electrons do not exist as far as I know. It is also possible to incorporate fluorescent protein markers into viruses for detection. This could enable, if anyone had that purpose, single virus detection by optical microscopy. (I added this to deal with the statement on single molecule detection by @S.McGrew.)
