Question

Calculate the number of photons of light of wavelength $3960 \,\mathring{A}$ necessary to provide $1\, J$ of energy.

• A. $2 \times 10^{20}$
• B. $2 \times 10^{16}$
• C. $2 \times 10^{15}$
• D. $2 \times 10^{18}$

Answer 1

Answer: (D)

Applying, $E=n \frac{h c}{\lambda}$

$E=1 J , h=6.6 \times 10^{-34} J s , c=3 \times 10^{8} m s ^{-1}$

$\lambda=3960 \mathring{A}=3960 \times 10^{-10} m$

$n=\frac{1 \times 3960 \times 10^{-10}}{6.6 \times 10^{-34} \times 3 \times 10^{8}}$

$=2 \times 10^{18}$