Final answer:
Penumbra or focal spot blur in wave optics refers to the diffusion of the focal point into a focal spot due to diffraction, with the size inversely proportional to the lens's numerical aperture.
Step-by-step explanation:
The term Penumbra, or focal spot blur, is related to the field of physics and more specifically in wave optics. It refers to the fact that the focal point of a light beam, due to diffraction through an optical system like a lens, does not converge into an infinitely small point but rather spreads out to form a 'focal spot'. This spot size is inversely related to the numerical aperture (NA) of the lens, meaning that a lens with a higher NA will have a smaller focal spot and therefore a greater intensity at that spot.
However, there is a trade-off as increasing the NA for higher intensity may result in greater chances of photodegrading the specimen being observed or analyzed due to the high intensity of light. In summary, a focal spot is never a true point, but a blurred spot with a size that is influenced by the lens's NA.