The human eye can barely detect a yellow light (6000 Å) that delivers 1.7 × 10–18 watt to the retina. Nearly how many photons per second does the retina receive ?

50 5 500 More than 5 million

5

$N_{ph}=\frac{P\lambda}{hc} =\frac{1.7\times 10^{-18}\times 6\times 10^{-7}}{6.63\times 10^{-34}\times 3\times 10^8}= \frac{10.2\times 10^{-25}}{20\times 10^{-26}} = 5.1$ $N_{ph}$ ≈ 5/sec