KEAM 2019 Physics and Chemistry Paper

Given, work function are as,
WNa=2.75eV
WCu=4.65eV
and ‌‌WAu=5.1eV
The frequency range of visible light 4×1014Hz to 8×1014Hz, so the maximum energy of photon and minimum energy of photon in visible light.
=3.31eV
Similarly, Emin=‌

6.63×10−34×4×1014

1.6×10−19

=1.65eV
Now, comparing the work function of above mentioned metal with Emax. We get only sodium device can give photoelectron.
Further sodium device can also work with photons of higher energy then 2.75eV like x -rays, UV rays etc.