Question

A small metal plate (work function $=1.17eV$ ) is placed at a distance of $2m$ from a monochromatic light source of wavelength $4.8\times 10^{-7}m$ and power 20 watt. The light falls normally on the plate. The number of photons striking the metal plate per second per sq metre area will be

• A. $9.65\times 10^{18}$
• B. $9.65\times 10^{17}$
• C. $9.65\times 10^{16}$
• D. $9.65\times 10^{15}$

Answer 1

Answer: (B)

Energy of photon emitted $=hc/\lambda$;
the energy of light falling per second per unit area of a metal surface
$=P/\left(4\pi r^2\right)$
$\therefore$ No. of photons falling per second per unit area of metal surface
$=\frac{P}{4\pi r^2}\times \frac{\lambda }{hc}$
$=\frac{20\times 4.8\times 10^{-7}}{4\times 3.14\times 2^2\times 6.6\times 10^{-34}\times 3\times 10^8}$
$=9.65\times 10^{17}$