Assessment
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1.
Download the simulation from http://phet.colorado.edu/en/simulation/photoelectric and open it.
Set the light color to red with the slider. Bring up the light intensity slowly toward 100%. Does the light have sufficient energy to dislodge
the electrons from the sodium metal? Do you see any blue dots representing electrons moving?
A) yes
B) no
2.
Use the Options menu (up & left) to select "Show Photons". Move the intensity slider back and forth. What is the intensity of light
actually controlling?
The number of photons
3.
Test with yellow light and green light. At what wavelength are the electrons first dislodged from the surface of sodium? Make sure to
write your units.
about 530 nm
4.
The wavelength that you just found corresponds to a frequency called the threshold frequency (using the relation speed = wavelength x
frequency). For sodium, the threshold frequency is 5.66 x 1014 Hz. With the color slider at the threshold frequency (or threshold
wavelength found in Q 3), what does changing the intensity do in the experiment?
A) more electrons are ejected
B) less electrons are ejected
C) nothing changes
5.
With the intensity at 50, slide the color slider to shorter wavelength (blue or smaller). What effect does higher frequency (shorter
wavelength) have on the ejection of the electrons?
The electrons eject at higher and higher speed.
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6.
Return the color slider to the color that just barely ejects electrons from the metal (Question 3). What is the energy of the photon in
electron-volt (ev)? Look at the course content website to find the formula for the energy of a photon. You can approximate Planck's
constant h to be 4 x 10-15 ev s and the threshold frequency f is approximately 6 x 1014 Hz.
A) 0.24 ev
B) 2.4 ev
C) 24 ev
D) 24 x 102 ev
7.
Slide the wavelength to 430 nm (violet light) (approximately f = 7 x1014 Hz). Calculate the energy of this photon in ev.
A) 1 ev
B) 2.8 ev
C) 2.8 x 10² ev
D) 400 ev
8.
Keeping the wavelength at 430 nm. Notice the speed at which electrons are now ejected from the surface. Where did this energy come
from? How much kinetic energy do they have (in ev)? I am a looking for a number and an explanation.
the extra energy is because each photons has more energy. 2.8ev -2.4 ev = 0.4 ev.
So the electrons are ejected with an extra 0.4 ev amount of energy which is all kinetic.
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9.
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The simulation makes watching electrons easy but in real life we don't see electrons. In order to figure out what is going on we put the
sodium inside of a vacuum tube and we connect the cathode and anode to a voltage source. Lets play with that a bit. Set the intensity to
50% and the wavelength to 200 nm (ultraviolet). Starting with zero voltage, slide to + 5 Volt. What do you observe regarding the motion
of electrons. What is the current in the circuit? The number in the bottom right part is given in amps.
13-10-24 11:12 AM
Assessment
https://cms.psu.edu/Section/Assessment/Delivery/AssessmentAl...
They go even faster although the current does not change as we increase the voltage. The current is
always 0.848 Amps
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10. With the wavelength held at 200 nm, ajust the electric potential difference (by adjusting voltage V) between the plates to just barely stop
the electrons. Record this value and write it below. This is the stopping potential.
the current stops at -4 V
11. Explain in words the flow of energy from the light to the electron. How can we figure out the energy of an individual photon by simply
measuring the stopping potential?
If we assume that light is made of photon each with energy E=hf, then when a photon hits an
electron it give all the energy to the electron. Some of it is used to get out of the metal (about
2.4 ev for sodium) and the rest is kinetic energy. I can figure out how much kinetic energy they
have when I know the stopping potential K =e times stopping potential. So once I know the kinetic
energy of the electron I can figure out the energy of the original photon.
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12. Choose one part of the photo-electric experiment that cannot be explained by assuming that light is a wave. Why do we need the
concept of light as a particles? (look at course website for some help there).
The stopping potential does not change when I change the intensity. It seems that the energy of the
electrons does not care on the energy of the wave. This make sense if light is made of particle.
The electron interact with just one photon. The fact that there are more photons don't change the
energy of each electrons.
Also threshold frequency. There is no reason to have a threshold frequency unless light is made of
particles whose energy depend on frequency.
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Done
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