Question

Sodium has a work function of 2.46 eV.

(a) Find the cutoff wavelength and cutoff frequency for the photoelectric effect.

wavelength | _________ nm |

frequency | _________ Hz |

(b) What is the stopping potential if the incident light has a
wavelength of 168 nm?

_________________V

Answer #1

(a) As given work function = 2.46 eV = 2.46 x 1.6 x
10^{-19} J

Work function = 3.936 x 10^{-19} J

Photon must have at least this much energy for knocking out an electron.

Hence, hf = 3.936 x 10^{-19}

h = planks const

f = frequency

f = 3.936/6.64 x 10^{-19+34}

f = 5.927 x 10^{14} Hz (this is required cut off
frequency)

As lambda = c/f = 3.0/5.927 x 10^{8-14}

lambda = 5.06 x 10^{-7} m (which is required cut of
wavelength)

(b) energy of incedent photon E = hc/lambda =( 6.64 x 3 / 168 )
x 10^{-34+8+9}

E = 1.185 x 10^{-18} J

As elenrgy of electron will be Ee = (11.85 - 3.936) x
10^{-19} = 8.484 x 10^{-19} J

As this must be equal to energy of electron in potential,

Ee = eV

V = Ee/e = 8.454 x 10^{-19}/1.6 x 10^{-19} V

V = 5.28375 Volt

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