Past exam of the physics 2 paper of the University of Oxford / 2014 resit / 10 / 4 / Solution

In wave optics, a finite lens aperture produces diffraction. Contributions interfere constructively near the geometric focus but form a finite point spread function, rather than a point of infinite intensity. For a uniformly illuminated circular aperture of diameter $D$, the paraxial focal pattern is an Airy disk, with first dark-ring radius approximately $1.22\lambda f/D$. Aberrations can broaden or distort it further; even an ideal lens retains the finite diffraction scale.