PHYS 3650L - Modern Physics Laboratory

PHYS 3650L= - Modern Physics Laboratory

Laboratory Advanced Sheet
Photoelectric Effect

1. Objectives. The objectives o= f this laboratory are

a. to verify the dependence of stopping potentia= l on the wavelength of light,

b. to determine Planck's constant, and

c. to measure the work function and threshold frequency of cesium antimonide.

2. Theory.

a. In 1887, during his electromagnetic= wave experiments, Heinrich Hertz discovered that under certain conditions light striking a material causes electrons to be emitted - a phenomenon known as = the photoelectric effect. Detailed investigation of the photoelectric effect conducted by Wilhelm Hallwachs and Philipp Lena= rd during the years 1886-1900 revealed some puzzling features that could not be understood on the basis of classical optics. In particular, it was found th= at:

i. W= hen the monochromatic light fell on the surface of the material, no photoelectrons were emitted unless the frequency of light was greater than = some minimum value called cutoff frequency (f₀) which depended= on the material.

ii. When the frequency f was gr= eater than the cutoff frequency, some electrons were emitted with significant amo= unt of kinetic energy.

iii. The kinetic energy of photoelectr= ons was dependent on the frequency of the light but not on the light intensity, while the magnitude of the photoelectric current was dependent on the inten= sity of light.

b. In 1905, Albert Einstein provided t= he explanation of photoelectric effect. Using an assumption proposed earlier by Max Planck, Einstein postulated that a beam of l= ight consists of small packages of energy called photons with the energy of a ph= oton equal to

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where

E is the energy of a photon,
f is the frequency of light, and
h is Planck's constant with the numerical value of

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A photon arriving at a surface is abso= rbed by an electron. If this energy is greater than the work function of the material (W₀), the electron may escape the surface. Einst= ein then applied conservation of energy to find the maximum kinetic energy for = an emitted electron given that electron gained energy = hf from a photon:

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K_max c= an be determined experimentally by measuring the stopping voltage DV₀:

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3. Apparatus and experimental proce= dures.

a. Equipment.

1) h/e apparatus with the accessory ki= t.

2) Mercury vapor light source.<= /p>

3) Three filters - yellow, green, and variable transmission filter.

4) Multimeter.

b. Experimental setup. To be provided = by the student.

c. Capabilities. To be provided by the student.

d. Procedures. Detailed instructions a= re provided in paragraph 4 below.

4. Requirements.

a. In the laboratory.

1) Your instructor will introduce you = to the equipment to be used in the experiment.

2) Measurements to determine the stopp= ing voltage as a function of intensity of light will be made for two different spectral lines from a mercury light source.

3) Measurements to determine the stopp= ing voltage will be made for five different spectral lines from a mercury light source (three trials each).

4) Your instructor will discuss method= s to be used to prepare your data for plotting using the Microsoft Excel^TM spreadsheet program.

b. After the laboratory. The items listed below = will be turned in at the beginning of the next laboratory period. A complete laboratory report is not required for this experiment.

Para= 3. Apparatus and experimental procedures.

1) Provide a figure of the experimental apparatus (para 3b).

2) Provide descriptions of the capabil= ities of equipment used in the experiment (para 3c).<= /span>

Para= 4. Data. Data tables are included at Annex A for record= ing measurements taken in the laboratory. A copy of these tables must be includ= ed with the lab report. Provide the items listed below in your report in the f= orm a Microsoft Excel^TMsprea= dsheet showing data, calculations and graphs. The spreadsheet will include:=

1) A table with columns for color of t= wo lines, % transmission, and measured stopping voltage.

2) A graph of the measured stopping potential versus %transmission.

3) A table with columns for color of l= ine responsible, corresponding frequency of the line, measured stopping voltage (three trials), and the mean stopping voltage.

4) A graph of the measured mean stoppi= ng voltage versus frequency of the line responsible showing data points and regression (trend) line. Include the equation of the trend line on your gra= ph.

5) Calculation of the value of Planck's constant.

6) Calculation of the value of work fu= nction of cesium antimonide.

7) Calculation of threshold frequency.=

8) Calculation of the percent discrepa= ncy in the Planck's constant.

Para= 5. Results and Conclusions.

5. Results and Conclusions.

a. Results.

1) A statement regarding the agreement= or disagreement between the predicted and measured dependence of stopping potential and the frequency of light.

2) A statement of the measured value f= or the Planck's constant and its percent discrepancy

3) A statement of the measured value f= or the work function of cesium antimonide and its comp= arison with the accepted value.

4) A statement of the determined thres= hold frequency.

b. Conclusions.

1) Assess the accuracy of your experiment.

2) Describe the sources of error in the experiment.

Annex A
Data
1. Stopping voltage vs %Transmission.

    a. Color 1.<= /p>

Color

%Transmission

Stopping Voltage, DV₀
(V)

100

80

60

40

20

      &n= bsp;            = ;

    b. Color 2.<= /p>

Color

%Transmission

Stopping Voltage, DV₀
(V)

100

80

60

40

20

      &n= bsp;            = ;

2. Stopping voltage vs frequency of light.

Color

Stopping Voltage, DV₀
(V)

Frequency, f (H= z)

Trial 1

Trial 2

Trial 3

Yellow

Green

Blue

Violet

UV

Color	Stopping Voltage, DV₀ (V)	Frequency, f (H= z)
	Trial 1	Trial 2	Trial 3
Yellow
Green
Blue
Violet
UV