# Suppose $10^{-17}J$ of light energy is needed by the interior of human eye to see an object .The photons of green light ($\lambda =550nm)$ needed to see the object are :

$(a)\;27\qquad(b)\;28\qquad(c)\;29\qquad(d)\;30$

$E_{needed}=10^{-17}=n\large\frac{hC}{\lambda}$
When n is no of photons needed of energy $\large\frac{hC}{\lambda}$ to provide energy of $10^{-17}J$
Hence (b) is the correct answer.