A light wave described by E = 60 [ (3 1015 t ) + (12 1015 t ) ] (in SI units) falls on a metal surface of work function 2.8 eV. The maximum kinetic energy of ejected photoelectron is (approximately) eV. (h = 6.6 10-34 J s and e = 1.6 10-19 C)
- 3.8
- 7.8
- 6.0
- 5.1
Answer
(4) 5.1
Solution
The wave contains two angular frequencies: 1 = 3 1015 rad s-1 and 2 = 12 1015 rad s-1.
The maximum kinetic energy is given by the photons of the higher frequency:
Eph = h = h 22 = 6.6 10-34 12 10152 3.14 J
Eph = 1.26 10-18 J = 1.26 10-181.6 10-19 eV 7.88 eV
(Photons of 1 have energy 1.97 eV < 2.8 eV, so they cannot eject electrons.)
By Einstein's photoelectric equation:
K = Eph - 0 = 7.88 - 2.8 5.1 eV
Answer: (4)