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Young’s experiment presents a simple example of interference between two crossing waves, actually providing the first demonstration of the wave nature of light. Two closely spaced narrow slits, A and B in Figure 4298a, transmit two elements of a monochromatic light wave from a single source. The two sets of waves spread due to diffraction effect, then overlap and interfere. They will be light and dark bands across the illuminated patch, resulting in interference fringes on the screen as shown in Figure 4298a.
Figure 4298a. Schematic illustration of Young’s experiment.
The fringe contrast, sometimes called visibility (V), is defined in terms of the maximum and minimum intensities as,
 [4298a]
Figure 4298b shows geometrical construction of Young's experiment. Constructive
or destructive interference occurs at P according to whether the path difference l is Nλ or (N+1/2)λ. Then, we can obtain that the distance between two adjacent bright fringes is given by Δy,
Δy = λL/d  [4298b]
similarly, the distance between two adjacent dark fringes is also λL/d. That is the bright and dark fringes are of equal width and are equally spaced. Furthermore, Equation 4298b indicates that fringe spacing increases with wavelength.
Figure 4298b. Geometrical construction of Young's experiment.
Information limit for HRTEMs is the inverse of the maximum spatial object frequency. The information limit is often obtained from measurements of diffractogram or from Young’s fringes provided a weak phase object scattering beyond the inverse of the information limit.
