Under a light microscope, can you see viruses?
Because viruses are small. The diameter of most individual virus particles is about 100nm. In other words, it would take about 100,000 virus particles to be arranged together to barely see it with the naked eye.
Viruses are so small that most viruses must be observed with the help of an electron microscope. The resolution of an electron microscope is 1,000 times that of an optical microscope. Sizes vary greatly between viruses. The smallest, such as plant Geminiviruses, are only 18-20nm in diameter, the largest, animal poxviruses, are 300-450nm × 170-260nm, and the longest, such as Filoviridae virus particles, are 18-20nm in diameter. 80nm×790-14000nm.
The resolution distance of the optical microscope is d=0.61λ/NA where
d--Resolving distance of the objective lens, unit nm.
λ--Wavelength of illumination light, unit nm.
NA --Numerical aperture of objective lens
For example, the numerical aperture of an oil immersion objective lens is 1.25, the visible light wavelength range is 400-700nm, and the average wavelength is 550 nm, then d=270 nm, which is approximately half the wavelength of the illumination light. Generally, the resolution limit of a microscope illuminated by visible light is 0.2μm, which is 200nm, which is larger than the diameter of the virus, so the virus cannot be seen with an optical microscope.
Bacteria are much larger than viruses. The diameter of a single cocci is about 0.8-1.2 μm. Most bacilli are medium-sized, 2-5 μm long and 0.3-1 μm wide, within the observable range of an optical microscope.
