Answer:
Since electron ejection is difficult from copper than sodium, so copper has greater work function than sodium. Work function, \[{{W}_{0}}=h{{\upsilon }_{0}}=\frac{hc}{{{\lambda }_{0}}}\] i.e., \[{{\lambda }_{0}}\propto \frac{1}{{{W}_{0}}}\] As threshold wavelength is inversely related with work function, so sodium has higher threshold wave- length than copper.
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