Question

How many photons of light having a wavelength $4000\mathring{A}$ are necessary to provide $1.00J$ of energy?

Answer 1

Energy of one photon $=hv=h\cdot \frac{c}{\lambda }$

$=\frac{(6.62\times {10}^{-34})(3.0\times {10}^8)}{4000\times {10}^{-10}}$

$=4.965\times {10}^{-19}J$

Number of photons $=\frac{1.00}{4.965\times {10}^{-19}}=2.01\times {10}^{18}$

Hence, $2.01\times {10}^{18}$ photons of light having a wavelength 4000A˚ are necessary to provide 1.00J of energy