A test flash of 5 minutes of arc is shown for 20 ms, and a test flash of 15 minutes of arc is shown next, also for 20 ms. Each emits 100 quanta. Which test flash is seen?

• A. 5 minutes of arc only
• B. 15 minutes of arc only
• C. Both
• D. Neither

Answer: (A)

Detection of a brief flash depends on how the photons are distributed across photoreceptors and whether any single receptor’s response crosses its threshold. Even though both flashes emit the same total quanta, the smaller spot concentrates those quanta into a much smaller retinal area. That high photon density per receptor makes it likely that at least one photoreceptor reaches the threshold needed to signal the brain, so the flash is perceived. The larger spot spreads the same 100 quanta over a wider region, lowering the amount of light hitting any single receptor. If no receptor reaches its threshold, the signal fails to rise above noise, and the flash isn’t seen. So the smaller angular size is detectable, while the larger one is not.