by, Sean Reynolds and Anne Ploetz
PASCO scientific
Model AP-9368 and AP-9369
1
Describe the effect that passing different amounts of the same colored
light through the Variable Transmission Filter has on the stopping potential and thus the
maximum energy of the photoelectrons, as well as the charging time after pressing the
discharge button:
As we passed different amounts of the same color of light through the h/e
Apparatus, we found a relationship of the amount of light vs. maximum energy, the more
light we let in, the more energy we measured. We found an inverse relationship between
amount of light and charging time. We found that when the energy was higher, the time
to charge was much lower, and as energy decreased the time to charge grew dramatically.
2
Describe the effect that different colors of light had on the stopping potential and
thus the maximum energy of the photoelectrons.
As we passed different colors of light through the h/e Apparatus, we noticed that
green and yellow had a lower stopping potential or maximum energy than blue and
ultraviolet light did. Because this is related to the time to charge, green and yellow had
longer times to charge than ultraviolet did, even though we used the same level on the
Variable Transmission Filter.
3
Defend whether this experiment supports a wave or quantum model of light based
on your lab results.
Light as a wave would show that energy of light is found to be relative to the
wave length or frequency of that light. So the experiments using different colors of light
should show different energy levels of light as different colors are measured. It did.
Light as a quantized particle would support the idea of getting different energy
levels based off of how much light is let in to the h/e Apparatus. Thus the experiment
using different settings of the Variable Transmission Filter would give different energy
levels for our measured values of the stopping potential. It did.
Although to be viewed as a wave it should exclude getting different energy
readings from the filters, and to be viewed as a particle, the color of light should not
matter for energy, the experiment showed light must be viewed as both wave and particle.
I think it’s clear that Light is a particle and because the particle is so small it feels the
effects of quantum mechanics, which says that everything very small has wave
properties. Thus you cant simply call light a particle, you must also think of it as a wave.
4
Explain why there is a slight drop in the measured stopping potential as the light
intensity is decreased.
Since light is a particle that waves, if you decrease the intensity that simply means
you do not allow as many particles into the h/e Apparatus. This means that there are less
particles to measure and less particles to wave. So as you add a filter you should see a
difference in not only the same color of light but in different colors of light as well.
Analysis: Part Two
1
Determine the wavelength and frequency of each spectral line. Plot a graph of the
stopping potential vs. frequency.
Graph is in Data Section.
2
Determine the slope and y-intercept. Interpret the results in terms of the h/e ratio
and the Wo/e ratio. Calculate h and Wo.
Stopping Potential
Slope = h/e = 4.04e-15
0
frequency
y=
-1.28
This graph portrays the results from graphing frequency versus the stopping potential.
The slope is
4.04e-15. The y intercept on the graph is -1.28. Using the equation KEmax = eVo, we can
find that the KEmax of yellow light is .811V(1.602e-19) = 1.3e-19. Plugging this into the
equation of
hf = eVo + Wo. Using the given for h, we can find that Wo is equal to 2.14e-19. The
Wo/e ratio is equal to 1.34. Each electron is using this much energy to free itself from the
metal and make the jump over to the other side, creating a current along the way.
To calculate the h/e ratio, we can take the given h and the given e. This given is very
close to the value that we received. The value that we should have received was 4.136e15. Our value was 4.04e-15. The percent difference is 2%. To find h, or plank’s
constant, we take h/e which is equal to 4.04e-15. Under our calculations and results, h
should equal 6.464e-34, which also is a percent difference of 2%. Our variances from the
given value come from the improper readings off of the plate inside the box. It was
difficult to line up the beam of light perfectly into the box. The small differences that we
have could be due to that.
3
In your discussion, report your values and discuss your results with an
interpretation based on a quantum model for light.
Values reported in Data Section. The stopping potential we found for the second
order is less than the stopping potential for the first order for the various colors. The
orders of light are caused by the lens in front of the mercury light source. The second
order is the second set of reflected light patterns. The second source is dimmer than the
first order. Because it is dimmer we can tell with our eyes, there are less photons making
up the light, or its intensity is decreased. Thus under the quantum model for light, there
are less photons reaching the h/e Apparatus, so less voltage is read by the DMM.
Spectrum broken by Lens Grating Assembly
Tech info on h/e Apparatus
GREEN
100%
80%
60%
40%
20%
STOPPING POTENTIAL
.923V
.921V
.917V
.908V
.895V
CHARGE TIME
14.53 S
23.84 S
25.71 S
61.47 S
117.88 S
BLUE
100%
80%
60%
40%
20%
STOPPING POTENTIAL
1.506V
1.506V
1.507V
1.504V
1.499V
CHARGE TIME
5.11 S
10.07 S
6.60 S
16.11 S
19.05 S
LIGHT COLOR
YELLOW
GREEN
BLUE
VIOLET
ULTRA VIOLET
COLOR
YELLOW
GREEN
BLUE
VIOLET
ULTRA VIOLET
STOPPING POTENTIAL
.815V
.937V
1.521V
1.768V
2.079V
STOPPING POTENTIAL
FIRST ORDER
.811V
.94V
1.511V
1.72V
2.02V
STOPPING POTENTIAL
SECOND ORDER
.750V
1.007V
1.518V
1.692V
2.01V
2
FIRST ORDER
STOPPING
POTENTIAL VS.
WAVE LENGTH
1.8
1.6
1.4
1.2
5.5
6.5
7
7.5
8
6.5
7
7.5
8
0.8
SECOND ORDER
STOPPING
POTENTIAL VS.
WAVE LENGTH
2
1.8
1.6
1.4
1.2
5.5
0.8