Light with an energy flux of 20 W/cm2 falls on a non-reflecting surface at normal incidence. If the surface has an area of 30cm2, the total momentum delivered during 30 minutes is a) 36 × 10-5 kg m/s b) 36 × 10-4 kg m/s c) 108 × 104 kg m/s d) 1.08 × 107 kg m/s
The intensity of the light incident is given by: I=AtE Here, I=20W/cm2, Area A=30cm2, time t=30mins=30×60sec=1800sec ∴20=30×1800E ⇒E=20×30×1800 Now, E=λhc and λ=ph E=phhc=p×c ⇒p=cE Momentum, p=3×10820×30×1800kgm/s=36×10−4kgm/s
