• # question_answer Light with an energy flux of 18 $W/c{{m}^{2}}$ falls on a non-reflecting surface at normal incidence. The surface has an area of 20 $c{{m}^{2}}$, then the total momentum delivered on the surface during a span of 30 min is A)  $2.16\times {{10}^{-3}}kg-m/s$ B)  $1.52\times {{10}^{-5}}kg-m/s$ C)  $8.31\times {{10}^{-8}}kg-m/s$   D)  $18.2\times {{10}^{-6}}kg-m/s$

Total energy falling $U=(18\,W/c{{m}^{2}})\,(30\times 60s)\,(20c{{m}^{2}})$ $=6.18\times {{10}^{5}}J$ Total, momentum delivered is $P=\frac{U}{c}=\frac{6.48\times {{10}^{5}}}{3\times {{10}^{8}}}=2.16\times {{10}^{-3}}kg-m/s$