Question

Suppose ${10}^{-17}$ J of light energy is needed by the human eye to see an object. How many photon of green light $(\lambda =550nm)$ are needed to generate this minimum amount of energy?

Answer 1

Energy of a single photon $E=\frac{hc}{\lambda }=\frac{6.626\times {10}^{-34}\times 3\times {10}^8}{550\times {10}^{-9}}=3.61\times {10}^{-19}J$

Number of photon required $=\frac{1\times {10}^{-17}}{3.61\times {10}^{-19}}=28photons$