OBJECT:
To study the emission of electrons from a metal surface which is irradiated with light. To experimentally determine the value of Planck's constant "h" by making use of the spectral dependency of the photoelectric effect.
APPARATUS:
A Mercury Vapor Light Source, a special h/e apparatus with a photodiode tube, a digital voltmeter, and associated light filters.
BACKGROUND:
In 1901 a German physicist Max Planck published his law of radiation. Planck went on to state that the energy lost or gained by an oscillator is emitted or absorbed as a quantum of radiant energy, the magnitude of which is expressed by the equation:
E = h n
where E equals the radiant energy, n is the frequency of radiation, and h is a fundamental constant, now known as Planck's constant. Albert Einstein applied Planck's theory and explained the photoelectric effect in terms of the quantum model using his famous equation for which he received the Nobel Prize in 1921:
E = hn = KEmax +f
where KEmax is the maximum kinetic energy of the emitted photoelectrons, and f is the energy needed to remove them from the surface of the material (the work function). Here E is the energy supplied by the quantum of light known as a photon.
In the h/e experiment, light photons with energy h n are incident upon the cathode of a vacuum tube. The electrons in the cathode use a minimum f of their energy to escape, leaving the surface with a maximum energy of KEmax. Normally the emitted electrons reach the anode of the tube, and can be measured as the photoelectric current. However, by applying a reverse potential V between the anode and cathode, the photoelectric current can be stopped. KEmax can then be determined by measuring the minimum reverse potential needed to bring the photoelectric current to zero. Thus, Einstein's relation becomes:
hn = Ve + +f
When solved for V, the equation becomes:
V = (h/e) n - ( f /e)
Thus, a plot of V versus n for different frequencies of light will yield a linear plot with a slope (h/e) and a V intercept of (- f /e).
SETUP PROCEDURE:
Experiment 1: Wave Model versus Quantum Model
According to the photon theory of light, KEmax for photoelectrons depends only on the frequency of the incident light, and is independent of intensity. Thus the higher the frequency, the greater its energy.
In contrast, the classical wave model of light predicted that KEmax would depend on light intensity. In other words, the brighter the light, the greater the energy.
This lab investigates these assertions. The experiment selects two spectral lines from a mercury line source and investigates KEmax versus intensity. The different spectral lines show if KEmax results with different light frequencies.
Procedure
|
Color #1 (name) |
|
%Transmission | Stopping Potential | Approx. Charge Time |
100 |
|
|
80 |
|
|
60 |
|
|
40 |
|
|
20 |
|
|
|
Color #2 (name) |
|
%Transmission | Stopping Potential | Approx. Charge Time |
100 |
|
|
80 |
|
|
60 |
|
|
40 |
|
|
20 |
|
|
Analysis
Experiment #2: Determining Planck's Constant
In this experiment you will select different spectral lines from mercury and investigate the maximum energy of electrons as a function of the wavelength and frequency of the light.
Procedure:
Color |
Stopping Potential First Order |
Stopping Potential Second Order |
Yellow |
|
|
Green |
|
|
Blue |
|
|
Violet |
|
|
Ultraviolet |
|
|
Analysis:
COLOR | WAVELENGTH |
Yellow | 579.0 nm (average) |
Green | 546.1 nm |
Blue | 435.8 nm |
Violet | 404.7 nm |
Ultraviolet | 365.5 nm |