Final answer:
The ideal shape for a focal spot is as small as possible to create high intensity, but due to diffraction, it cannot become a perfect point. The focal spot size decreases and intensity increases with the objective lens's numerical aperture. Parabolic mirrors help in achieving a well-defined focal point.
Step-by-step explanation:
The ideal shape for a focal spot would be one that is as small as possible to achieve high intensity, which increases the resolution of the image produced. However, due to the limitations of wave optics and the presence of diffraction, the focal point becomes a focal spot. The size of the focal spot decreases with an increase in the numerical aperture (NA) of the objective lens, which in turn increases the intensity within the spot. Nevertheless, because of diffraction limitations, the spot can never become a perfect point. Using a parabolic mirror can help in creating a well-defined focal point if the mirror is small compared to its radius of curvature, as this approximates a perfect parabolic shape that directs all parallel rays to the same point after reflection. In contrast, a spherical mirror, if large compared to its radius of curvature, will suffer from spherical aberration, causing a blurred image due to the lack of a well-defined focal point.