Consider focusing when only considering geometric optics, shown in Figure 7a. One of the consequences of diffraction is that the focal point of a beam has a finite width and intensity distribution.
Thus the microscope has more information to form a clear image, and so its resolving power will be higher. Another way to describe this situation is that the larger the NA, the larger the cone of light that can be brought into the lens, and so more of the diffraction modes will be collected. Lenses with larger NA will also be able to collect more light and so give a brighter image. A lens with a large NA will be able to resolve finer details. In a microscope, NA is important because it relates to the resolving power of a lens. Telescopes are also limited by diffraction, because of the finite diameter D of their primary mirror. So diffraction limits the resolution of any system having a lens or mirror. Thus light passing through a lens with a diameter D shows this effect and spreads, blurring the image, just as light passing through an aperture of diameter D does. Be aware that the diffraction-like spreading of light is due to the limited diameter of a light beam, not the interaction with an aperture. The acuity of our vision is limited because light passes through the pupil, the circular aperture of our eye. There are many situations in which diffraction limits the resolution. This limit is an inescapable consequence of the wave nature of light. If they were closer together, as in Figure 1c, we could not distinguish them, thus limiting the detail or resolution we can obtain. The pattern is similar to that for a single point source, and it is just barely possible to tell that there are two light sources rather than one. How does diffraction affect the detail that can be observed when light passes through an aperture? Figure 1b shows the diffraction pattern produced by two point light sources that are close to one another. (c) If they are closer together, they cannot be resolved or distinguished. (b) Two point light sources that are close to one another produce overlapping images because of diffraction. (a) Monochromatic light passed through a small circular aperture produces this diffraction pattern.
0 kommentar(er)
