Past exam of the physics 2 paper of the University of Oxford / 2021 / 9 / b / Solution

The screen now lies in the back focal plane of the lens. In paraxial geometrical optics, a light ray arriving at the lens at height $y$ and angle $\theta$ leaves at angle $\theta -y/f$, and reaches the screen atThus all light rays with the same incident direction reach the same screen coordinate, independently of their height at the lens. The Fraunhofer diffraction pattern is an angular pattern mapped to $x=f\theta$:Here $I_{s,f}$ is the one-slit light intensity after focusing. Its absolute value depends on collected light and the lens aperture, which are unspecified; it need not equal the no-lens value $I_s$. Propagation from the slits to the lens introduces a common angular wave phase, so it does not alter the relative wave phase responsible for these fringes, provided the aperture passes the required light.