Introduction to QP

Photoelectric effect

Light quantum (photon) = h



Wave or particle?

L3-1

• •

Photoelectric effect

What is photoelectric effect?

Who discovered it?

Hertz (1887) Lenard (1902) •

Who explained it?

Einstein (1905) •

Controversy: wave or particle?

•

Final Experimental proof

Millikan (1913-1914) •

Applications

Introduction to QP L3-2

The remarkable aspects - observation

• • Electromagnetic radiation can push electrons free from the surface of a solid. o This process is called the photoelectric effect. o o A material that can exhibit the photoelectric effect is said to be photoemissive. Electrons ejected by the photoelectric effect are called photoelectrons. The photoelectric effect will not occur when the frequency of the incident light is less than the threshold frequency. o Different materials have different threshold frequencies. o Most elements have threshold frequencies in the ultraviolet region of the electromagnetic spectrum. Introduction to QP L3-3

• The maximum kinetic energy of a stream of photoelectrons … o o o is determined by measuring the stopping potential: the applied voltage needed keep the photoelectrons trapped in the photoemissive surface. increases linearly with the frequency of the incident light above the threshold frequency. is independent of the intensity of the incident light. • The rate at which photoelectrons are emitted from a photoemissive surface … o is determined by measuring the electric current. o is directly proportional to the intensity of the incident light when frequency is constant. • On a graph of maximum kinetic energy vs. frequency … o o all curves are linear with slope equal to Planck's Constant. the intercept on the energy-axis is the threshold frequency of the material. Introduction to QP L3-4

The energy of individual ejected electrons increases linearly with the frequency of the light!

K max = F(f)

Introduction to QP L3-5

• Classical physics cannot explain why … o no photoelectrons are emitted when the incident light has a frequency below the threshold, o o o the maximum kinetic energy of the photoelectrons increases with the frequency of the incident light, the maximum kinetic energy of the photoelectrons is independent of the intensity of the incident light, and there is essentially no delay between absorption of the radiant energy and the emission of photoelectrons. • Modern physics explains it o photoelectric effect:

K

max = E − ϕ = h(f − f 0 ) o electron energy:

K

max = eV 0 Introduction to QP L3-6

Introduction to QP Work Functions for Photoelectric Effect Element Aluminum Beryllium Cadmium Calcium Carbon Cesium Cobalt Copper Gold Iron Lead Magnesium Mercury Nickel Niobium Potassium Platinum Selenium 3.68

4.5

5.01

4.3

2.3

6.35

5.11

Work Function( eV) 4.08

5.0

4.07

2.9

4.81

2.1

5.0

4.7

5.1

4.5

4.14

Silver Sodium Uranium 4.73

2.28

3.6

Zinc 4.3

Handbook of Chemistry and Physics

L3-7

The controversy: wave or particle?

The photoelectric effect suggested a particle nature for light!

The energy of the ejected electrons was proportional to the frequency of the illuminating light: the interaction must be like that of a particle which gave all of its energy to the electron! James Clerk Maxwell’s wave theory of light: to predict that the electron energy would be proportional to the intensity of the incident light.

Introduction to QP L3-8

Resources

http://www.nobelprize.org/educational/physics/quantised_world/ http://www.faqs.org/docs/qp/chap01.html

Introduction to QP http://physics.about.com/od/quantumphysics/p/quantumphysics.htm

http://galileo.phys.virginia.edu/classes/252/photoelectric_effect.html

http://hyperphysics.phy-astr.gsu.edu

http://www.colorado.edu/physics/2000/quantumzone/photoelectric.html

http://physics.info/photoelectric/ Introduction to QP L3-9