25. To find the longest possible wavelength λ
max
(corresponding to the lowest possible energy) of a photon
which can produce a photoelectric effect in platinum, we set K
max
= 0in Eq. 39-5 and use hf = hc/λ.
Thus hc/λ
max
= Φ. We solve for λ
max
:
λ
max
=
hc
Φ
=
1240eV
·nm
5.32 eV
= 233 nm .