Question

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?

• A. 50
• B. 5
• C. 500
• D. More than 5 million

Answer 1

Answer: (B)

5

Answer 2

n_­Ph = 5 × 10²⁴ P l/sec

­­

W m

= 5 × 10²⁴ (1.7 × 10^–18) (6000 × 10¹⁰)

= 5 × 1.7 × 6 × 10^{27 – 28}

= 51 × 10^–1 = 5.1/sec

n_Ph $12$ 5/sec