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Perhaps there need not be “degrees of quantumness”. As the beams pass increasingly closer to the surface, the plate will induce a small (but increasingly larger) spread of energies (hence wavelengths) in the electrons within the beam, possibly explaining the “smearing out” of the fringes in the interference pattern.

Michael D. Meloy
Santa Barbara, Calif.

Peter Sonnentag of the University of Tübingen in Germany says that the loss of energy is tiny: less than 0.1 percent. Moreover, Sonnentag says that the change in energy could account for a change only in fringe spacing, not in fringe visibility .—D. Castelvecchi