1. The Photoelectric effect

2. Unespected results
• The emission of electrons depends on
frequency
• energy of electrons generated by dim
illumination is the same as that generated
by bright illumination so, NO
DEPENDANCE ON LIGHT INTENSITY
(electrons have the same speed)
• It is a contradiction if compared to
basic wave theory
• (classically energy is proportional to wave
amplitude squared)

3. Proof
• Stopping potential applied to the anode:
(reverse potential to prevent
electrophotons to reach the anode)
• KE = ½ mv2
= eVs
• It was shown that Vs is directly
proportional to the maximum kinetic
energy of the photoelectrons and does
not depend on Intensity (same V when
intensity changes)

4. Previous results used by
Einstein
• Planck had showed that radiation is
emitted in pulses of energy hf
• E = hf

5. Einstein explanations
-
Some energy is needed
to release the electron
(the work function φ)…
…and some energy is
given to the electron as
kinetic energy.
Photon Energy = work function + kinetic energy of electron

6. Photoelectric Effect: Vstop vs. Frequency
stopeV hf φ= −
min0stopV hf φ= ⇒ =
Slope = h = Planck’s constant
hfmin
−φ

7. Photoelectric Effect: Vstop vs. Frequency
stopeV hf φ= −
min0stopV hf φ= ⇒ =
Slope = h = Planck’s constant
hfmin
−φ