Question

The human eye can barely detect a yellow light $(\lambda =6000\stackrel{o}{A})$ that delivers $1.7\times {10}^{-18}$ W to the retina. The number of photons per second falling on the eye is nearest to

• A. $5\times {10}^9$
• B. 5000
• C. 50
• D. 5

Answer 1

The correct option is D 5

Energy of the yellow light is $E=\frac{hc}{\lambda }=\frac{(6.62\times {10}^{-34})\times (3\times {10}^8)}{6000\times {10}^{-10}}=3.31\times {10}^{-19}J$

Here the total energy per sec is given $1.7\times {10}^{-18}W$

Thus, number of photons per sec $=\frac{1.7\times {10}^{-18}}{3.31\times {10}^{-19}}\sim 5$