Question

The number of photons of light of wavelengths $7000\mathring{A}$ equivalent to $1J$ are :

• A. $3.52\times {10}^{-18}$
• B. $3.52\times {10}^{18}$
• C. $50,000$
• D. $10,0000$

Answer 1

Answer: (B)

$3.52\times {10}^{18}$

Energy, $E=\frac{hc}{\lambda }$

Energy of a $single$ photon of wavelength 7000 Angstrom is $\frac{hc}{7000\times {10}^{-10}}$

put $h=6.6\times {10}^{-34}Joule-second$ and $c=3\times {10}^{8}m/s$

we get energy of a single photon as $E=2.829\times {10}^{-19}Joule$

Number of photon in $1Joule=\frac{1J}{E}=3.52\times {10}^{18}$

Option B is correct