(II) Suppose a thin piece of glass is placed in front of the lower slit in Fig. 24-7 so that the two waves enter the slits 180° out of phase (Fig. 24-58). Draw in detail the interference pattern seen on the screen.
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When there is no glass piece in front of two slits, there is no phase difference between the waves passing through slits. Two crests or two troughs interfere at the center point of the screen. So at the center point of the screen, bright fringe occurs as in the figure.