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## Homework Statement

A multi-akali alloy is used as the photocathode material in a photoemissive electron tube. The work function is 1.6eV a blue light of wavelength 420nm is incident to the photocathode. What should be the voltage and polarity to extinguish the current

## Homework Equations

kintetic energy=hf-hf0

## The Attempt at a Solution

To my understanding a negative voltage with the same magnitude as the work function is needed to extinguish the current completely, and therefore the voltage should be -1.6eV, however,the solution says that the voltage needed to extinguish current is -1.36eV which is the kinetic energy

thanks for reading