Question

Light of frequency 2.5 x 1015 Hz illuminates a

piece of iron, and the iron emits photoelectrons
with a maximum kinetic energy of 6.3 eV. What is
the threshold frequency of the metal? Planck's
constant is 6.63 x 10-34 J·s.
.

Answer 1

h f = W + KE

Input energy equals work function plus KE of emitted electron

W = 6.63E-34 * 2.5E15 - 6.3 * 1.6E-19

W = 6.63 * 2.5 * 10^-19 - 10.1 * E-19 ev (1ev = 1.6E-19 J)

W = (16.6 - 10.1)E-19 = 6.5E-19 J

h f = 6.5E-19 J for electrons to be emitted with zero KE

f = 6.5E-19 / 6.63E-34 = .98E-15 / sec = 9.8E-14 / sec (threshold)