Question

A beam of white light incident normally on a plane surface absorbing 70% of the light and reflecting the rest . if the incident beam carries 10W of power , find the force exerted by it on the surface . (1) 4.3× 10-5 N. (2) 4.3 × 10-6N. (3) 4.3 × 10-7N. (4) 4.3×10-8N

Answer 1

Answer on Question #75367, Physics / Quantum Mechanics

A beam of white light incident normally on a plane surface absorbing 70% of the light and reflecting the rest. If the incident beam carries 10W of power, find the force exerted by it on the surface. (1) $4.3 \times 10^{-5} \mathrm{~N}$. (2) $4.3 \times 10^{-6} \mathrm{~N}$. (3) $4.3 \times 10^{-7} \mathrm{~N}$. (4) $4.3 \times 10^{-8} \mathrm{~N}$

**Solution:** $P = 10\mathrm{W}, P = \frac{h\nu}{\Delta t} = \frac{hc}{\lambda\Delta t}$

The force

$$F = \frac {\Delta p}{\Delta t} = \frac {0 . 7 \frac {h}{\lambda} + 0 . 6 \frac {h}{\lambda}}{\Delta t} = \frac {1 . 3 h}{\lambda \Delta t} = \frac {P}{c} = \frac {1 . 3 \cdot 1 0}{3 \cdot 1 0 ^ {8}} = 4. 3 \cdot 1 0 ^ {- 8} N.$$

**Answer:** (4) $4.3 \times 10^{-8} \mathrm{~N}$

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