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Q: 11.5 The energy flux of sunlight reaching the surface of the earth is  1.388\times 10^3 \hspace{1mm}W/m^2. How many photons (nearly) per square metre are incident on the Earth per second? Assume that the photons in the sunlight have an average wavelength of  550 \hspace{1mm}nm?

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Average Energy(E) of the photons reaching the surface of the Earth is given by

\\E=\frac{hc}{\lambda }\\ E=\frac{6.62\times 10^{-34}\times 3\times 10^{8}}{550\times 10^{-9}} \\E=3.61\times 10^{-19}\ J

Energy flux(I) reaching the Earth's surface=1.388\times103 Wm-2

Number of photons(n) incident on Earth's surface per metre square is

\\n=\frac{I}{E}\\ n=\frac{1.388\times 10^{3}}{3.61\times 10^{-19}}\\ n=3.849\times 10^{21}\ m^{-2}

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