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Photoelectric Effect

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Subject
Chemistry
School
San Diego State University
Type
Homework
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1-6: Photoelectric Effect
Although Albert Einstein is most famous for E = mc
2
and for his work describing relativity in mechanics,
his Nobel Prize was for understanding a very simple experiment. It was long understood that if you
directed light of a certain wavelength at a piece of metal, it would emit electrons. In classical theory, the
energy of the light was thought to be based on its intensity and not its frequency. However, the results of
the photoelectric effect contradicted classical theory. Inconsistencies led Einstein to suggest that we need
to think of light as being composed of particles (photons) and not just as waves. In this experiment, you
will reproduce a photoelectric experiment and show that the energy (E) of a photon of light is related to its
frequency and not its intensity.
1. To start this activity, click the link given in the assignment instructions on Canvas.
2. At the top left of the laboratory bench you will see a laser light source with displays for intensity and
wavelength of the light produced by the laser. To control the laser, go to the Live Data window and
scroll down. You will see a slide bar that you can click to switch the laser on/off. Below that you will
see the controls for intensity and wavelength:
Intensity can be set to 1 MW (1 Mega Watt), 1 kW, 1 W, 1 mW, 1 W, 1 nW, 1000 p/s (photon
per second), 100 p/s, 10 p/s or 1 p/s (Note at 400 nm, 1 W is about 2 × 10
18
p/s). To change, click the
up ( ) or down (˅) arrows.
The wavelength control display has up ( ) or down (˅) arrows that allow you to increase or
decrease the wavelength by 1, 10 or 100 units, or to change the unit. You can also simply select one
or all digits and type in a number.
Towards the middle of the bench you will see a metal foil target (hover your cursor over it to identify
the metal) and in the bottom left you will see a phosphor screen detector.
When the laser light source is switched on, a beam of light will be directed at the metal foil target. If
an electron is ejected from the metal it can be detected when it strikes the phosphor screen. You see a
view of the phosphor screen in the Live Data window. You should a bright green dot in the center of
the phosphor screen (which is a dull green color). If you switch the laser off, the bright green dot
should disappear, and then reappear if you switch the laser back on.
3. Which metal foil is used in this experiment?
The metal foil used is the sodium (Na)
At what intensity is the laser set? At what wavelength is the laser set?
The laser is set to an intensity of 1 nW. The laser is set to a wavelength of 400 nm.
Record the wavelength (in nm) and color of the laser light in the data table on the following page.
Click on the Spectrum Chart (behind the laser); to help determine color based on wavelength.
Calculate the frequency (in Hz) and the energy (in J) for this wavelength. Show your work here and
record the answers in the data table on the following page. Show your answer to 3 significant figures.
c = 2.998 × 10
8
ms
-1
and h = 6.626 × 10
-34
Js.
f = c/v = 2.998 × 10
8
ms
-1
/ 4 × 10
-7
m = 7.495 × 10
14
Hz
E = hf = 6.626 × 10
-34
Js (7.495 × 10
14
Hz) = 4.966 × 10
-19
J

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4. Decrease the laser intensity to 1 photon/second (p/s). Describe the signal that you see on the phosphor
screen.
The signal on the phosphor screen is now blinking and has become dimmer.
5. Increase the laser intensity in steps to 1kW. How does the phosphor screen signal change?
The signal on the phosphor screen has become a steady signal now, unblinking and slightly
brighter than it was before.
6. Change the laser intensity back to 1 nW and increase the wavelength to 600 nm.
What do you observe?
The light has disappeared from the phosphor screen.
Calculate the frequency (in Hz) and the energy (in J) for this wavelength. Record along with the
wavelength and light color in the data table below.
f = c/v = 2.998 × 10
8
ms
-1
/ 6 × 10
-7
m = 4.997 × 10
14
Hz
E = hf = 6.626 × 10
-34
Js (4.997 × 10
14
Hz) = 3.311 × 10
-19
J
7. Determine the maximum (max) wavelength at which ejection of electrons occurs in the metal by
gradually changing the wavelength (use the 1, 10 and 100 unit places) and monitoring the signal on
the phosphor screen. Record the max wavelength and color of this light in the table blow.
Calculate the frequency (in Hz) and the energy (in J) for this wavelength. Record in the data table.
f = c/v = 2.998 × 10
8
ms
-1
/ 4.5 × 10
-7
m = 6.662 × 10
14
Hz
E = hf = 6.626 × 10
-34
Js (6.662 × 10
14
Hz) = 4.414 × 10
-19
J
Data Table
wavelength (nm)
frequency (Hz)
energy (J)
light color
400
7.495 × 10
14
Hz
4.966 × 10
-19
J
Violet
600
4.997 × 10
14
Hz
3.311 × 10
-19
J
Orange
(max) 450
6.662 × 10
14
Hz
4.414 × 10
-19
J
Blue
8. What is the difference between intensity and wavelength? Which matters in the ejection of electrons:
intensity or wavelength (frequency)?
The difference between light intensity and wavelength is that light intensity is concerned on the
amplitude of the wave with the same wavelength; meanwhile, wavelength is a light property that
determines the color of the light. Between these two, the ejection of electrons depends on the
wavelength of light, which will then determine the energy of the photons.
9. Click the Presets menu and select Photoelectric Effect (Bolometer). The intensity of the laser will be
set at 1 nW and the wavelength at 400 nm. The detector used in this experiment is a bolometer and
will be automatically turned on. This instrument measures the kinetic energy of electrons. You should
see a green peak on the bolometer detection screen. The height of the green peak corresponds to the
number of electrons being ejected from the metal, and the x-axis is the kinetic energy of the electrons.

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1-6: Photoelectric Effect Although Albert Einstein is most famous for E = mc2 and for his work describing relativity in mechanics, his Nobel Prize was for understanding a very simple experiment. It was long understood that if you directed light of a certain wavelength at a piece of metal, it would emit electrons. In classical theory, the energy of the light was thought to be based on its intensity and not its frequency. However, the results of the photoelectric effect contradicted classical theory. Inconsistencies led Einstein to suggest that we need to think of light as being composed of particles (photons) and not just as waves. In this experiment, you will reproduce a photoelectric experiment and show that the energy (E) of a photon of light is related to its frequency and not its intensity. 1. To start this activity, click the link given in the assignment instructions on Canvas. 2. At the top left of the laboratory bench you will see a laser light source with displays for intensity and wavelength of the light produced by the laser. To control the laser, go to the Live Data window and scroll down. You will see a slide bar that you can click to switch the laser on/off. Below that ...
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