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LAB 11
Photoelectric Effect & Blackbody Radiation
OBJECTIVES
1. Experimentally verify that the emission of photoelectrons depends only on
frequency of the incident light and not the intensity of the light.
2. Predict & measure Planck’s constant h/e and the work function Φ/e of a metal.
3. Examine the blackbody radiation emitted by objects at different temperatures.
EQUIPMENT
UV light source, aluminum plate, electric pom-pom, plastic rod, wool, Photoelectric effect
apparatus, DMMs, LEDs, PhET Photoelectric Effect Simulation.
THEORY
In photoelectric emission, light strikes a metal (cathode), causing electrons to get
emitted. As a guide, use the following PhET simulation:
http://phet.colorado.edu/en/simulation/photoelectric
The classical wave model predicts several things about the photoelectric effect: (i) the
total energy is spread across a wavefront striking the entire metal surface and nothing
could happen until sufficient energy were absorbed. Therefore, no photoelectrons would
be immediately ejected and one expects a time lag as to when light is absorbed and
photoelectrons are ejected. (ii) Increasing the intensity increases the kinetic energy of
the most energetic electrons (i.e. Kmax) would be emitted from the metal. (iii) Increasing
the intensity would also increase the number of electrons emitted (or photocurrent). On
the other hand, the quantum model predicts that (i′) photoelectrons are immediately
ejected if the frequency is high enough assuming fincident > fthresold. (ii′ & iii′) Increasing
intensity does produce a higher photocurrent but Kmax is independent of intensity.
Furthermore, Einstein applied Planck’s theory and explained the photoelectric effect
using conservation of energy:
h 
f  e
e
E  hf  K max    eVstop   
 Vstop  
where Kmax is the maximum KE of the emitted photoelectrons and  is the work function,
which is the metal’s binding energy. Plotting Vstop vs. f for different frequencies, the yintercept is equal to Φ/e and the slope is equal h/e.
PROCEDURE
Part 1: Photocurrent vs. incline voltage at different light intensities
Predict the behavior of the photocurrent (ejected electrons) & stopping voltage
a. Use the wave model to predict the following behaviors:
 Does the photocurrent immediately register a value when the light source is turn
on or is there a time lag?
 Is the photocurrent higher for higher intensity levels?
 Is the stopping voltage higher for higher intensities or independent of intensity?
b. Repeat (1a) using the photon model.
Measure Photocurrent vs. Vincline
c. Setup a data table with columns for Vincline(V) with values of (0, 0.2, 0.4, 0.6, 0.8, 1.0,
Vstop), photocurrents iphoto(A) with intensity levels 1, 2, 3, and 4.
d. Insert the blue LED into the photoelectric apparatus and cover the apparatus with the
cover. Set the intensity level to 1, turn on the incline voltage & set to 0V, and
measure the photocurrent. Increase Vincline to 0.2V, measure the new photocurrent,
and record this value. Repeat this process until Vincline reaches the Vstop value.
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e. Repeat (1d) for intensity levels 2, 3, and 4. Plot iphoto vs. Vincline for all four intensities
on a single plot.
f. Using your observations and the plot, answer the questions of (i) time lag, (ii) Vstop
(Kmax) ∝ Intensity, and (iii) iphoto ∝ intensity , and based on the predictions of the
 Wave model
 Quantum model
 Which model better fits the data better: the wave or quantum model? Explain
your answer using short concise sentences.
Part 2: Predict and measure Planck’s constant h/e and the Work Function Φ/e
a. Setup a data table with columns for LED description, wavelength,
frequency, and stopping voltage Vstop.
b. Set the intensity level to 4 (maximum intensity) and measure Vstop for the
LEDs UV, blue, green, yellow, red and IR.
a. Plot Vstop vs. frequency and use the Trendline function to display the line
equation. Determine (h/e)expt and (Φ/e)expt from the line equation and write
your values on the whiteboard.
b. Compute the class average and uncertainty ((h/e)class ± 2σh). Does the
accepted value of Planck’s constant fall within 2σh confidence interval,
where (h/e)accepted = 4.14 ×10-15 V∙s?
c. What is the work function/e ≡ (Φ/e of the metal (cathode) plate? Performing an
online search of work functions, are there any metals with this value? Explain your
reasoning.
Part 3: The Photoelectric Effect and the Electric Pom-pom
Photons hitting a metal surface (aluminum) will cause electrons to be ejected if the work
function is less than the photon energy: Ephoton = hf > Φ. Will ultraviolet (UV) light cause
electrons to be ejected from an aluminum surface?
a. The UV light source produces two different wavelengths, 365 nm and 254 nm.
Calculate the photon energy in electronvolts (eV) for each of these two wavelengths
using hc = 1240 eV∙nm.
b. The work function for aluminum is ΦAl = 4.28 eV. Predict if electrons will be ejected
by the UV light source. Explain you’re reasoning in terms of Ephoton, ΦAl and an
energy bar diagrams. Record all information in a table.
c. Place the aluminum surface on top of the electric pom-pom and use a charged rod to
scrap electrons from the rod onto the metal surface. When the strings of the pompom are repelled from each other, then the metal surface has an excess of electrons.
i.
Shine UV 365 nm light onto the aluminum surface; are electrons ejected from it?
ii.
Repeat (i) for UV 254 nm light.
d. Did your predictions from part (1b) agree with what you observed? Explain your
observations in terms of the photoelectric effect using short concise sentences.
Part 4: Blackbody Radiation
All objects above absolute zero emit electromagnetic radiation called blackbody
radiation. As an object gets hotter, it emits more light and it emits light of higher energy
(thus higher frequency and shorter wavelength). Warm objects emit mostly infrared light,
IR, which we can’t see, but this special camera can “see” infrared light.
In this station, you will observe the visible and infrared radiation emitted by three
hotplates at different temperatures.
a. Use the camera and your eyes to answer the following questions:


Which emits more IR light – your eyes, or your nose? Why?
Look at a cup of hot water vs a cup of cold water. Which one emits more infrared
light? Why?
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b. Look at the wire coil in the camera.
 Tap your foot quickly on the switch. Explain the types of light do you see?
 Now hold down the switch and let the coil get hot. Does it emit visible light? Is it
still emitting infrared light as well?
 If we heat up something even more, what will happen to the wavelengths of light
it emits?
c. Does IR light passes through these materials (Predict first!): (i) Clear acrylic, (ii)
glass, (iii) black plastic, and (iv) balloon.
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