Monochromatic light incident on a metal surface emits electrons with kinetic energies from zero to 2.6 eV. What is the least energy of the incident photon, if the tightly bound electron needs 4.2 eV to remove?

6.8 eV

Maximum KE = 2.6 eV,

Work function, $\phi_0$ = 4.2eV

Energy of incident photon = Maximum KE + $\phi_0$

= 2.6 + 4.2 = 6.8 eV