A beam of electromagnetic radiation of… - JEE Main 2020 Physics PYQ

JEE Main 2020 — Physics Modern Physics

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A beam of electromagnetic radiation of intensity $6.4\times { 10}^{ -5} \text{W}/ { \text{cm}}^{2}$ is comprised of wavelength, $\lambda =310nm$ . It falls normally on a metal (work function $\phi =2eV$ ) of surface area of $1{cm}^{2}$ . If one in ${10}^{3}$ photons ejects an election, total number of electrons ejected in $1s$ is ${10}^{x}$ . $(hc=1240eVnm,1eV=1.6\times {10}^{-19}J),$ then $x$ is ___________

Official previous-year question

Held on 7 Jan 2020 · Verified 6 Jul 2026.

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Solution

Energy of photon. $E=\frac{1240}{310(\text{nm})}=4eV>2eV$ (So photoelectric effect will take place)

$=4\times 1.6\times {10}^{-19}=6.4\times {10}^{-19}joule$

Number of photons falling per second

$=\frac{6.4\times {10}^{-5}\times 1}{6.4\times {10}^{-19}}={10}^{14}$

Number of photoelectron emitted per second

$=\frac{{10}^{14}}{{10}^{3}}={10}^{11}$

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