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—y 4.5 DIFFRACTION AT A CIRCULAR OPERATURE Since a lens passes only that light which falls with in its circular perimeter, it behaves like a circular aperture in an opa - a W screen, Such an aperture forms a diffraction pattern similar ¢, th : at | of a single slit. Diffraction effects often limit the ability of optic instruments to form precise images. The quality of image forme, by a lens is also affected by chromatic and spherical aberration, which are, however, minimised or eliminated by adopting differen, methods e.g. shaping of the lens surfaces or using lens combination _with suitable separation. However, it is not easy to eliminate diffraction effects which are determined by the lens diamete; (aperture) and wavelength of light used. In diffraction, nature imposes a fundamental limitation on the precision of optical instruments. In geometrical optics we do not care for diffraction effects. However, in wave-optical analysis of the formation of a image by a lens, diffraction effects arise in a natural way. The image of a distant point source of light such as star formed on q. photographic film placed in the focal plane of the converging lens of a telescope is shown in Fig. (4:15) called the diffraction pattern of a | circular aperture. 
