1. The Particle of Light
A particle model of light is necessary to describe phenomena observed in modern physics, for example, the interaction between light and atoms.
PHYS140
Light as a Particle

2. The Photoelectric Effect
Many physicists’ work contributed to the discovery of the photoelectric effect
What is it?
The ability of light to dislodge electrons from a metallic surface
The electrons can be detected and the resulting signals amplified
Lots of applications in visual imaging
PHYS140
Light as a Particle

3. Questions How many electrons are ejected in a given time?
How does this number depend of wavelength or intensity?
How energetic of the ejected electrons?
Upon what does the electron energy depend?
Are electrons ejected instantly or is there a time delay?
PHYS140
Light as a Particle

4. Photoelectric Experiments
Cathode – electrons are ejected
Anode – electrons are collected
Experiment 2
Experiment 1
PHYS140
Light as a Particle

5. Photoelectric Experiments - con’t
a) Electrons freely flow from the anode back to the cathode and they are counted along the way
Can determine how # of e- depends on wavelength and intensity; time light must shine on cathode for electrons to flow
PHYS140
Light as a Particle

6. Photoelectric Experiments - con’t
b) Ejected electrons have to overcome the electric field to get to the anode
PHYS140
Light as a Particle

7. Photoelectric Experiments - con’t
b) Ejected electrons have to overcome the electric field to get to the anode
Can determine energy of ejected electron
If the potential difference between the plates, ΔΦ = 2.0 V, the difference between the electron’s electrostatic potential energy at the anode and its potential energy at the cathode is
The electron can make it to the anode only if it has an initial kinetic energy greater than this
PHYS140
Light as a Particle

8. Wave Model Predictions
The rate at which electrons are ejected from a metal is proportional to the intensity of the incident light.
Lower intensity light rays should have a delay before electrons are ejected
The rate may depend on frequency (wavelength) of light
The maximum kinetic energy of the electrons is likely to increase with increasing intensity
PHYS140
Light as a Particle

9. Experiments Provide the Following Results
At high intensities and fixed frequencies, the # of ejected electrons is proportional to intensity
Electrons are ejected instantly, regardless of intensity level
For constant intensity, the # of electrons decreases with increasing frequency
If the frequency is below a certain level, no electrons are ejected, regardless of intensity level
Above the cutoff frequency, the electrons’ maximum kinetic energy is propostional to the frequency of light ✔
Green check - agrees with wave predictions
Red check – doesn’t
PHYS140
Light as a Particle

10. Maximum Energy depends on Frequency
Above the cutoff frequency, the electrons’ maximum kinetic energy is proportional to the frequency of light
Green check - agrees with wave predictions
Red check – doesn’t
PHYS140
Light as a Particle

11. Einstein’s Prediction – light is a particle
Light consists of particles, each carrying a certain amount of energy
Where E is the energy, f is the frequency, and h is Planck’s constant
We typically express colors of light in wavelengths
Green check - agrees with wave predictions
Red check – doesn’t
PHYS140
Light as a Particle

12. Einstein’s Prediction - con’t
Einstein also predicted that each electron ejected from the metal was a result of a collision with a single photon
Where K is the kinetic energy of the electron and W is the work function for the metal
The work function is the energy required to liberate the electron from the metal
Green check - agrees with wave predictions
Red check – doesn’t
PHYS140
Light as a Particle

13. Einstein’s Prediction - con’t
Einstein’s model explains the experimental results so neatly, why was there resistance in the science community?
This model is completely inconsistent with the wave nature of light.
Neither model, wave or particle, adequately explains light by itself
Green check - agrees with wave predictions
Red check – doesn’t
PHYS140
Light as a Particle

14. Extra Credit
Several extra credit projects have been added to the schedule.
Check Moodle and the online schedule for more info.
Acousto-magnetic strips deter shoplifters, due 4/23
Blue Man Group Pipe Instruments, due 4/23
Make an instrument, due 4/28
Applications of the photoelectric effect, due 4/28
PHYS140
Light as a Particle

15. Practice
Interactive activity – photoelectric effect for different metals
Go to the site below and answer the questions at the bottom of the page
Interactive problem
Group Problems
Q3B.5, Q3S.4
PHYS140
Light as a Particle