1. The problem statement, all variables and given/known data In an experiment a laser beam is focussed on a sample by a lens. The sample has micro structures spatially separated by 5 microns. In the experiment, each micro-structure has to be studied individually using the laser, so that illumination of two or more adjacent micro-structures must be avoided. If the laser wavelength is 632 nm and the diameter of the beam is 1 mm, find focal lengths of the lens suitable for this experiment. 2. Relevant equations I have no idea. 3. The attempt at a solution I assumed that the spot sized needs to be focussed to >5 microns so it meets the requirements. but I really have no idea.