First slide

Photo Electric Effect

Question

When a 25 watt point source of power, emitting mono chromatic light of wavelength $4000 \overset{o}{A}$ is kept at a distance of 50 cm from a photo sensitive surface of work function 2 eV. Then stopping potential needed to stop the most energetic emitted photo electrons is

Easy

A

1.1 volt
B

1.6 volt
C

2.4 volt
D

3.1 volt

Solution

Energy of individual photon, $h γ = \frac{h c}{λ} = \frac{12420}{4000} e V = 3.105 e V$
$∴$ Maximum kinetic energy of photoelectrons
$= h γ = ϕ = (3.105 - 2) e V = 1.105 e V$
$∴$ Stopping potential = 1.1 volt

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