Diffraction occurs when a wave bends around objects. Diffraction is also said to occur when a wave spreads after it has passed through a type of gap.
Huygens' principle states that waves can spread in spheres or circles when in two-dimensional and three-dimensional space. Diffraction is the reason why a microscope is unable to be used to see much smaller details. In general, microscopes are only able to see details that are bigger than the used wavelength. Optical microscopes are unable to be used for any objects that are smaller than a micrometer. Viruses are mostly smaller than a micrometer and are usually undetected by microscopes.