Page 79 - Fundamentals of Light Microscopy and Electronic Imaging

62 DIFFRACTION AND INTERFERENCE IN IMAGE FORMATION

Figure 5-1
Diffraction image of a copper mesh grid. A 400-mesh grid was illuminated with a laser
pointer, and its diffraction pattern was photographed on a projection screen. Multiple
diffraction spots are observed due to the relatively large spacing between grid bars and the
coherent laser light source.

water waves incident on a log floating in a pond. The waves wrap around the ends of the
log into the geometrical shadow; instead of reflecting away from the ends of the log,
they seem to grab hold of the ends and swing themselves around into the sheltered zone.
The redirected component of diffracted light is easily observed when a tree or person is
backlighted by a strong light source under conditions where the background behind the
object is still dark; the bright line outlining the silhouette of the object is diffracted light.
Of particular interest to us is the image of a point source of light in the microscope, since
images are composed of a myriad of overlapping points. As we will see, waves emanat-
ing from a point in the object plane become diffracted at the margins of the objective
lens (or at the edges of a circular aperture at the back focal plane of the lens), causing
the image of the point to look like a spot. Thus, the image of a point in a microscope is
not a point at all, but a diffraction pattern with a disk of finite diameter. Because of dif-
fraction, an object’s image never perfectly represents the real object and there is a lower
limit below which an optical system cannot resolve structural details.
Diffraction is also observed when a beam of light illuminates a microscope slide
covered with fine dust or scratches (Fig. 5-2b). The spreading of an automobile’s head-
light beams on a foggy night is another good example of the phenomenon. In these
cases, diffraction is defined as the scattering of light by small particles having physical
dimensions similar to the wavelength of light. The amount of scattering and angle of