Sunlight of mean wavelength $550 nm$ and intensity $1.5 kW m^{- 2}$ falls normally on a surface of area $1.0 c m^{2}$ . Find the number of photons arriving per second in units of $1 0^{17}$ , to two decimals. Use $h = 6.626 \times 1 0^{- 34} J s$ and $c = 3.0 \times 1 0^{8} m s^{- 1}$ .

Numerical answer type, answer in x 10^17 photons/s, tolerance 0.03.

Answer: 4.15 x 10^17 photons/s

Power on the area = 1500 x 1.0e-4 = 0.15 W. Energy per photon = hc/

λ

= 3.6142e-19 J. Photons per second = 0.15/3.6142e-19 = 4.150e+17 = 4.15 x

1 0^{17}