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When a light of photons of energy 4.2 eV is incident on a metallic sphere of radius 10 cm and work function 2.4 eV , photoelectrons are emitted. The number of photoelectrons liberated before the emission is stopped , is $(e=1.6 \times 10^{-19}C\: and \: \large\frac{1}{4 \pi \varepsilon_{\circ}} = 9 \times 10^9\: N-m^2/Coulomb^2)$ :

$\begin {array} {1 1} (1)\;6.25 \times 10^8 & \quad (2)\;1.25 \times 10^9 \\ (3)\;1.25 \times 10^8 & \quad (4)\;6.25 \times 10^{18} \end {array}$

(3) $1.25 \times 10^8$