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 30 cm2 the total momentum delivered (for complete absorption) during 30 minutes is ______.

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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 30 cm2 the total momentum delivered (for complete absorption) during 30 minutes is `underline(36 xx 10^-4 kg m/s)`. Explanation: Given, energy flux phi = 20 W/cm2 Area, A = 30 cm2 Time, t = 30 min = 30 $\times$ 60 s Now, total energy falling on the surface in time t is U = phiAT = 20 $\times$ 30 $\times$ (30 $\times$ 60) J Momentum of the incident light = U/c = `(20 xx 30 xx (30 xx 60))/(3 xx 10^8)` = 36 $\times$ 10–4 kg ms–1 Momentum of the reflected light = 0 $\therefore$ Momentum delivered to the surface = 36 $\times$ 10–4 – 0 = 36 $\times$ 10–4 kg ms–1 As no reflection from the surface and for complete absorption so the momentum of reflected radiation is zero. Momentum delivered to surface = Change in momentum = `p_f - p_t = 0 - 36 xx 10^-4 = - 36 xx 10^-4` kg m/s (–) sign shows the direction of momentum.

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