Question

The number of red photons (l = 663 nm) that must strike a totally reflecting screen per second at normal incidence so that a force of 1 N is exerted on the screen is

• A. n = 5 × 10²³
• B. n = 5 × 10²⁴
• C. n = 5 × 10²⁵
• D. n = 5 × 10²⁶

Answer 1

Answer: (D)

n = 5 × 10²⁶

Answer 2

F = n_ph $\left( \frac{2h}{\lambda} \right)$

n_ph = $\frac{F\lambda}{2h}$ = $\frac{1 \times 663 \times 10^{–9}}{2 \times 6.63 \times 10^{–34}}$