
8
DIFFRACTION
Let us begin this chapter by considering a laser pointer with a beam 1 mm in diameter, as
shown in the left side of Figure 8.1. If we point it at a wall a few centimeters away, the beam
will still be about the same size as it was at the exit of the pointer. Even a few meters away,
it will not be much larger. However, if we point it toward the moon, the beam diameter will
be very large and no noticeable light would be detected by an observer there. At what angle,
α, does the beam diverge? At what distance is the divergence noticeable? Can we control
these numbers? Geometric optics offers us no help, as it predicts that a collimated beam ...