Fig. 7 Schematic diagram showing Wheeler's Delayed-Choice Experiment. Source S emits a single photon in direction of two slit apparatus. On passing slits, photon is either absorbed by emulsion E if it is up. If emulsion is down, photon conti nues on to detection in either telescope T1 or telescope T2 which are focused on slits. Operator decides position of emulsion after photon has passed through slit system.