The Photoelectric Effect

Textbook: 12.1

Homework: pg. 608 #2, 8, 12 - 14

Planck’s Constant

In 1900, German physicist Max Planck suggested that light travels in packets called quanta.

These packets define the amount of energy transferred by a given frequency of light. E = hf h = 6.63 x 10-34 Js

The main postulate of quantum mechanical theory:

E is the smallest amount or quantum of energy (in joules) that can be transferred for a given wavelength of electromagnetic radiation.

hcE

The Photoelectric Effect

Current flows if frequency of light is greater than threshold frequency (f0)

Work function, W, is the minimum amount of energy needed for the electrons to be emitted from metal surface.

Einstein received the Nobel Prize in physics in 1921

Photons

Light is made of particles called photonsEphoton = hf

h = 6.63 x 10-34 Js 1eV = 1.6 x 10-19 J = 1V

Electron requires energy to escape surface of metal, left over energy provides Ek

Ek = hf - WW = Work Function

Pg 604 11. Calculate the minimum frequency of the

photon required to eject electrons from a metal whose work function is 2.4 eV.

13. Barium has a work function of 2.48 eV. What is the maximum kinetic energy of the ejected electrons if the metal is illuminated at 450 nm?

15. Light of frequency 8.0 x 1014 Hz illuminates a surface whose work function is 1.2 eV. What is the maximum speed with which an electron reaches the plate?

Applications

charge-coupled device (CCD) a semiconductor chip with an array of light-sensitive cells, used for converting light images into electrical signals

Solar panels

Photo-resistors

Digital cameras

Momentum of a Photon: The Compton Effect

1923: American physicist A.H. Compton directed a beam of high energy X-ray photons at a thin metal foil

Compton effect: the scattering of lower – frequency photons by high-energy photons

hp

Practice (pg 608 - 609)

1. Show that the units of are units of momentum.

2. Use Planck’s quantum theory to suggest a reason why no photoelectrons are released from a surface until light of sufficiently high frequency is incident on the surface.

8. Calculate the energy of an ultraviolet photon, of wavelength 122 nm, in both joules and electron volts.

h

13. What wavelength of light is required for ejecting photoelectrons from a tungsten surface (W = 4.52 eV) if the maximum kinetic energy of the electrons is 1.68 eV?

14. When light of wavelength of 482 nm falls onto a certain metallic surface, a retarding potential of 1.2V proves just sufficient to make the current passing through the phototube fall to zero. Calculate the work function of the metal.

16. (a) Calculate the momentum of a photon of wavelength 2.50 x 10-9 m.

(b) Calculate the speed of an electron having the same momentum as the photon in (a). (me = 9.11 x 10-31 kg).

(c) Calculate the kinetic energy of the electron. How does it compare with the energy of the photon?

21. (a) Calculate the energy of a single microwave quantum of wavelength of 10.0 cm.

(b) Calculate how many quanta of 10.0-cm microwave energy would be required to raise the temperature of 250.0 mL of water from 20°C to the boiling point, given that the specific heat capacity of water is 4.2 x103 J/kg?°C.