Page 81 - Fundamentals of Light Microscopy and Electronic Imaging

64 DIFFRACTION AND INTERFERENCE IN IMAGE FORMATION

Resultant wave

Constructive
0
interference

(a)

Resultant wave

Destuctive
0
interference

(b)

Figure 5-3
Two coincident waves can interfere if their E vectors vibrate in the same plane at their point
of intersection. Two waves are shown that vibrate in the plane of the page. In these
examples, both waves (dotted and dashed curves) have the same wavelength, but vary in
amplitude. The amplitude of a resultant wave (solid curve) is the arithmetic sum of the
amplitudes of the two original waves. (a) Constructive interference occurs for two waves
having the same phase. (b) Destructive interference occurs for waves shifted in phase; if the
amplitudes of the waves are the same and the relative phase shift is /2, the wave is
eliminated.

and interference in directions that permit constructive interference. Likewise, interfer-
ence filters do not destroy the light they do not pass; they merely reflect or absorb it. It
is therefore best to think of diffraction and interference as phenomena involving the
redistribution of light waves and photon energy. Accordingly, wave constructions of the
kind shown in Figure 5-3 are best thought of as devices that help us calculate the energy
traveling in a certain direction or reaching a certain point. The mechanism by which
light energy becomes redistributed is still not understood.

THE DIFFRACTION IMAGE OF A POINT SOURCE OF LIGHT

The image of a self-luminous point object in a microscope or any other image-generating
instrument is a diffraction pattern created by the action of interference in the image plane.