# Einstein's Theory Of The Photoelectric Effect

969 Words 4 Pages
Introduction
The photoelectric effect refers to the ejection or emission of electron from a metallic substance when said substance is exposed to electromagnetic radiation, such as photons of light. The ejected electrons are called photoelectrons.

The purpose of this lab is to use this phenomenon to determine an experimental value for h, Planck’s constant, the constant of proportionality between the energy and frequency of a single light quantum. It also verifies that Einstein’s theory of the photoelectric effect is more accurate than the classical theory (as compared to a competing theoretical explanation of the effect).

Theoretical Background
The photoelectric effect, first discovered in the late 1800’s by JJ Thompson, proposed that
1 Experimental Arrangement [Taken from Page 3 of the Photoelectric lab guidelines]

Experimental Procedure
Exercise 1: Insert an LED into the power supply box and align it with the phototube window Set up the two multimeters as voltmeters, to measure the stopping voltage and the photocurrent (which is measured as a potential drop across the 100 kΩ resistor) Turn the power supply and the potentiometer on. Measure the stopping voltage (Vstop) using the most sensitive range of the voltmeter for accuracy Repeat steps 1 to 3, for each of the 8 wavelengths (LED’s) provided

Exercise 2: Insert the variable intensity LED into the power supply box, align it with the phototube window, and set up the two multimeters as in Exercise 1 Use the variable intensity LED and independently measure the stopping voltage and the photocurrent as a function of intensity.

Exercise 3: Connect the phototube to Channel 1 of the oscilloscope through the rectifying adaptor. Ensure that the phototube power supply is not turned on. Set up the two multimeters as voltmeters, to measure the stopping voltage and the photocurrent (similar to Exercise 1) Connect the wave generator to the oscillator-driven LED and to channel 2 of the
Set the oscilloscope to trigger off Channel 2 and adjust the settings such that 1 to 4 full periods of oscillation can be seen within the viewing window. If needed, use Acquire → Average functions and a 20MHz filter. Measure the transient photocurrent as a function of time and estimate the delay between incident light and a response in the photocurrent.

Data and Calculations

Note that since at Vstop=0 no current was flowing; we can say the cutoff frequency of the particular material used in this experiment is equal or greater than that of Infrared light used in this experiment.
We exclude the infrared light in our calculations for that reason, and will evaluate the claim later.

Given that speed of light is used in the definition of SI units for length and time, we can assume its uncertainty zero. Therefore, the uncertainty of frequency can be calculated as such: σ_f/f=√((σ_c/c)^2+(σ_λ/λ)^2 ) σ_f=σ_λ/λ f
Where c is the speed of light, f is the frequency of light, λ is the wavelength of light and σ stands for uncertainty.
V_stop=h/e [f-f_0

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