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A 160 watt light source is radiating light of wavelength $6200 \stackrel{0}{A}$ uniformly in all directions. The photon flux at a distance of $1.8 \mathrm{~m}$ is of the order of (Plank's constant $\left.6.63 \times 10^{-34} \mathrm{~J}-\mathrm{s}\right)$ :
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$10^{19} \mathrm{~m}^{-2} \mathrm{~s}^{-1}$
$\begin{array}{ll} & \frac{\mathrm{N}}{4 \pi \mathrm{R}^{2}} & \mathrm{~N}=\frac{160 \times 6200 \times 10^{-10}}{6.62 \times 10^{-34} \times 3 \times 10^{8}} \\ & =\frac{5 \times 10^{19} \times 10}{4 \times 3.14 \times 3.24} & =\frac{16 \times 62 \times 10^{-7}}{20 \times 10^{-34} \times 10^{8}} \\ & =\frac{5 \times 10^{19} \times 10}{4 \times 3.14 \times 3.24} & =\frac{496}{10} \times 10^{19} \\ & =1.25 \times 10^{19} \end{array}$
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