H133: 1094 Session 2
Write your name and answers on this sheet and hand it in at the
There are a variety of activities today;
watch the time and try to get through them all.
Work with others at your table on these activities. Argue about
the answers but work efficiently!
After the indicated time, move on to the next activity!
1. Q3: The Photoelectric Effect [20 min.]
Start up the PhET applet "The Photoelectric Effect"
(Start->Programs->PhET, choose "Quantum Phenomena" from the left menu,
and click once on the Photoelectric icon).
The simulation shows light with a well-defined wavelength and
intensity (which you control), incident on a metal plate (cathode),
which causes the emission of electrons (depicted as little balls).
The electrons can travel (if they have enough energy) to another
plate (anode), completing an electric circuit. You can apply
a voltage and measure the current.
- Increase the intensity so that electrons flow.
Sketch here your prediction for what a graph of the current vs.
the light intensity would look like. Then select the "Current
vs light intensity graph" and move the intensity through its
entire range. Fix your graph as needed. Is this result consistent
with the wave model or particle model of light, or both? Explain.
- Set the intensity halfway.
Adjust the voltage to make the electrons just stop,
so the current drops to zero
(this is called the stopping voltage).
Why is the polarity of the voltage such that the anode is
How is the stopping voltage related to the maximum kinetic energy
of the electrons?
You can read off the wavelength of the light and the stopping voltage.
Calculate the work function for sodium (and give an appropriate
number of significant figures).
- Note that the initial target is Sodium. Devise a way using
the "Electron energy vs light frequency graph"
to rank the work functions of all the possible targets
from lowest to highest. Try it out. Explain what you did and
give your result.
(Hint: Move the wavelength through its entire range.)
2. Q4: The Wave Nature of Matter [15 min.]
Start up the PhET applet "Quantum Wave Interference".
This applets simulates wave phenomena from both light (photons)
and particles (electrons, neutrons, helium atoms).
- Turn on the light gun and then (on the right) click on the "Double
Slits". Look at the pattern on the screen. Predict below (qualitatively)
how it will
change when you change the slit separation, then test your
prediction with the "Slit Separation" slider.
- Now switch from photons to electrons (using the pulldown menu
above the Gun Controls). Explain the pattern you see now
(you may need to adjust the Screen Brightness).
Predict what will happen to the pattern when you decrease the
electron velocity to 700 km/s and then test your prediction.
- Do problem Q4B.2. (Hints: Follow Example Q4.1. 1 keV = 1000 eV.)
What will happen if a beam of such electrons passes through a
0.01 nm slit?
- Look at problem Q4R.2 (but don't solve it). What is the
effect of making h much larger than in our universe? Why are
there places to stand where you won't be hit by bats?
3. Group Problems [10 min.]
- Why do radio waves diffract around buildings, although light waves
The photograph shows the pattern on a screen from a distant source
of red light that passes through two very narrow slits. Point P is
the center, which is bright, and Q is another bright region. Point R
marks a minimum of intensity.
- Find the path difference in terms of the wavelength lambda
for each of the points P, Q, R.
- The distance from P to R is 1.6 mm and the screen is 2.2 m from
the slits. What is the slit separation?
- Give two reasons why the usefulness of large telescopes
increases as we increase the lens diameter.
H133: 1094 Session 2.
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