In the described experiments, how many quanta does each test flash emit?

• A. 50
• B. 100
• C. 200
• D. 400

Answer: (B)

Quanta are photons, so a test flash delivers a fixed number of photons to the retina on each trial. Emitting 100 quanta per flash provides a middle ground: it’s bright enough to be detectable on some trials but not so bright that detection is almost certain on every trial. This balance lets the experimenters chart a useful detection curve and estimate sensitivity. If you used only 50 quanta, detection would be rare and data would be noisy; with 200 or 400 quanta, detection would be almost guaranteed, giving less information about thresholds. So 100 quanta per flash is the best choice for measuring how sensitivity changes around the detection threshold.