7.4.1.1. Collimating the edge waves with a lens.
When the disk is placed in the primary focus of a postive lens,the spherical waves are collimated and the angle, \(\alpha\) remains constant. As a result the beam does not diverge anymore. Ofcourse the light passing the disk at larger distances from the edge will be focussed as well by the lens and will disturb the Bessel beam. This light can easily been blocked with an extra aperture surrounding the disk resulting in an annular slit.
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