CAIE A-Level Physics 22.2.2 Threshold Frequency and Wavelength

Practise defining threshold frequency and wavelength and deciding whether incident radiation is above or below the photoelectric emission threshold.

Exam points

define and interpret threshold frequency and threshold wavelength
determine whether radiation is above or below the threshold for emission

[Maximum number: 3]

(a)

Light from the lamp in (b) is incident, separately, on the surfaces of caesium and tungsten metal.

Data for the work function energies of caesium and tungsten metal are given in Fig. 10.1.

Fig. 10.1

Calculate the threshold wavelength for photoelectric emission from

[ 3 ]

(i)

caesium, nm

[ 2 ]

$\Phi=h c / \lambda_{0}$

\begin{aligned} \lambda_{0} & =\left(6.63 \times 10^{-34} \times 3.00 \times 10^{8}\right) /\left(1.4 \times 1.60 \times 10^{-19}\right) \\ & =890 \mathrm{~nm} \end{aligned}

(ii)

tungsten.

threshold wavelength =nm [1]

[ 1 ]

$\lambda_{0}=\left(6.63 \times 10^{-34} \times 3.00 \times 10^{8}\right) /\left(4.5 \times 1.60 \times 10^{-19}\right)$

=280 \mathrm{~nm}

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22.2.2—The terms threshold frequency and threshold wavelength question 1

Question (a)

Question (i)

Question (ii)