Threshold for observers to see a faint impression of light (quanta incident on the cornea) falls in which range?

• A. 10-20
• B. 30-50
• C. 200-300
• D. 54-148

Answer: (D)

Light detection hinges on enough quanta actually reaching and activating photoreceptors to produce a detectable signal. In practice, especially for a faint flash under dark-adapted conditions, you need on the order of tens to about a hundred photons to reach the retina (and cornea) to be perceptible, because many photons are lost or fail to trigger a sufficient neural response due to absorption, scattering, and the probabilistic nature of photoreceptor activation.

That’s why the range around roughly 50 to 150 quanta is the best fit. It matches the well-established findings that the threshold lies in the few tens to around a hundred photons range, depending on factors like flash duration and adaptation state. The alternative ranges are either too low, given ocular losses and receptor efficiency, or too high for typical faint-light detection.

So the observed threshold falls in about 54–148 quanta incident on the cornea, reflecting the combined effects of photon loss along the way and the stochastic decision process of visual signaling.