The Photoelectric effect

1. The Photoelectric effect In 1886 and 1887 Hertz conducted experiments designed to demonstrate the exististance of electromagnetic waves

2. The experiments succeeded….Thus Maxwell was correct, radio waves exist Ultraviolet light can induce sparks (photo-electrons) Electron Metal

3. 12 years later... • Planck solved the thermal spectrum “problem” • Derived “blackbody spectrum” • Atomic oscillators must exchange energy in discreet units: E = nhf

4. In 1905 young Albert Einstein attacked the problem... • Didn’t see any reason why oscillators should exchange quantized energy, unless: • The energy itself was quantized! • Assumed that Electromagnetic energy came in the form of packets (“light quanta”) with energy E = hf. • Imagine counting your piggy bank…why do you always end up with a whole number? • Suggested proof was in photo-electric effect

5. 10 years later, Milikan did it! Light source A V Adjustable Voltage

6. Were Hertz and Maxwell Wrong? • Photocurrent (I) proportional to intensity of source • Electron energy (eVs) proportional to frequency (f) • No current below cutoff frequency • Occurs instantly or not at all eVs Slope = h f

7. Classical expectations: • Oscillators gain energy over time • Electrons should be ejected eventually at any frequency (a time lag should exist) • Electron energy should depend on Intensity • Maxwell’s equations couldn’t be wrong... • Plank Nominated Einstein to the Prussian Academy of Science “In spite of his unreasonable theory of light quanta”

8. Of course Einstein was right... • Photons with energy hf strike the surface of a given material.. • Ejecting an electron with kinetic energy hf - W… • Where W is the work function of the substance (a binding energy), thus Ke = hf - W • Said Milikan: “ I spent ten years of my life testing that 1905 equation of Einstein’s. I was compelled in 1915 to assert its unambiguous verification in spite of its unreasonableness, since it seemed to violate everything we knew about the interference of light”