Experimental evidence that light is a wave:

1
Experimental evidence that light is a wave
2
Photoelectric Effect

• Shine light of particular wavelength/frequency/col
  or on metal plate C
• Electron ejected from plate C (photo-electron)
  with certain kinetic energy
• Number of photons that reach plate A per second
  gives current (read on ammeter)
• Potential difference (bias voltage) between
  plates C A adjusted until current is zero ? no
  electrons reach plate A
• Use conservation of energy to determine maximum
  initial kinetic energy of ejected photo-electron

3
ConcepTest 49 In a photoelectric effect
experiment, you can adjust the frequency and the
intensity of the incoming light. Consider the
following choices a) You want to
increase the kinetic energy of the ejected
electrons. What should you adjust? b) You
want to increase the number of ejected electrons.
What should you adjust?
Photo-Electric Effect Mysteries
1. Increase f
2. Increase I
3. Increase either
4. No effect
4
Einsteins Photoelectric Effect Solution
Photon hypothesis light is a PARTICLE
Millikans photo-electric effect data
h Plancks Constant h 6.63 x 10-34 Js 4.14
x 10-15 eVs hc 1240 eVnm ? VERY USEFUL
5
Interpreting Intensity
6
ConcepTest 50 You have a beam of light aimed
at a surface of fixed area A, and can measure the
amount of energy delivered to that area in some
fixed amount of time. Consider the following
choices
1. Energy increases
2. Energy decreases
3. No change
4. Not enough info
a) What happens if you decrease the
wavelength? b) What happens if you reduce the
number of photons per second?
7
More evidence for the particle nature of light
Compton (Scattering) Effect
Localized Photon interactions with Matter
8
Wave Nature of Particles de Broglies hypothesis
9
ConcepTest 51 You want to construct a
microscope to look at small objects. You could
build the microscope so that it uses photons or
electrons. You can produce photons with
momentum 6.63 x 10-26 Js/m 4.14 x 10-7
eVs/m. You can also produce electrons with
the same momentum. Which would give you better
resolution in your microscope?
1. Photons
2. Electrons
3. Both equally good
4. Neither any good