Physics

Problem

In a photoemission experiment, the maximum kinetic energies of photoelectrons from metals

P

Q

and

R

are

E_P

E_Q

and

E_R

, respectively, and they are related by

E_P = 2E_Q = 2E_R

. In this experiment, the same source of monochromatic light is used for metals

P

and

Q

while a different source of monochromatic light is used for the metal

R

. The work functions for metals

P

Q

and

R

are 4.0 eV, 4.5 eV and 5.5 eV, respectively. The energy of the incident photon used for metal

R

, in eV, is _____.