# u7 moodle ```Name:_____________________________
AP Physics 2
Unit 7: Modern Physics Moodle
1. The human eye can respond to as little as 10-18 J of light energy. For a wavelength at the peak of visual
sensitivity, 550 nm, how many photons lead to an observable flash?
2. A 200 W infrared laser emits photons with wavelengths of 2.0 x 10-6 m, and a 200 W ultraviolet laser emits
photons with wavelengths of 7.0 x 10-8 m. What is the energy of a single photon from each laser – which
wavelength of photon has more energy? Which laser is brighter (emits more photons every second)?
Moodle: If both lasers are turned on at the same time, how many total photons are emitted every second?
3. What is the longest wavelength of light that will emit electrons from a metal whose work function is 2.90 eV?
4. Two different monochromatic light sources, one yellow (580 nm) and one violet (425 nm) are used in a
photoelectric effect experiment. The metal surface has a threshold frequency of 4.5 x 1014 Hz. What is the work
function of the metal? What is the threshold wavelength of the metal? What is the kinetic energy of the
ejected electrons when violet light is incident upon the material? Moodle: As the violet light is more energetic
than the yellow, how many more electrons are ejected from the metal if a violet photon hits it than if a yellow
photon hits it?
5. In the photoelectric effect experiment, it is observed that no current flows unless the wavelength is less than
550 nm. What is the work function of this material? Plot a graph of Kinetic Energy of Ejected Electrons vs.
Frequency for this metal. Plot a graph of Kinetic Energy of Ejected Electrons vs. Wavelength for this metal.
Moodle: What stopping potential is required when light of wavelength 400 nm is incident on the material?
6.
A Ferrari with a mass of 1400 kg approaches a freeway underpass that is 12 m across (cement cylinders
shoulder the road). At what speed must the car be moving in order for it to diffract as it passes through this
“single slit?”
7. Neutrons can be used in diffraction experiments to probe the lattice structure of crystalline solids. If you predict
that the crystal has about 0.3 nm of space between atoms, at what speed should you project neutrons into the
solid?
8. Visible light incident on a diffraction grating with slit spacing 0.010 mm has the first maximum at an angle of 3.6&deg;
from the central peak. If electrons could be diffracted by the same grating, what electron velocity would
produce the same diffraction pattern as the visible light?
9. Calculate the energy released when tritium, 31𝐻, β- decays. Give your answer in MeV. Write nuclear reaction
equation.
10. The isotope 218
84𝑃𝑜 can decay either by Beta Minus or Alpha decay. What is the energy release in both?
11. Osmium 174 B+ decays. How much energy is released? Write the nuclear reaction equation. Give your answer
in MeV.
12. Plutonium 240 decays into Uranium 236. Write the nuclear reaction equation and determine the energy
13. After a decay similar to #12, the Uranium remains in an excited state. So, it releases a gamma ray of energy 150
MeV. Determine the mass of the excited Uranium (before it released the gamma decay). Write the nuclear
14. By what fraction does the mass of a Hydrogen atom decrease when it makes an n = 3 to an n = 1 transition? The
mass of ground state hydrogen is 1.00794 amu’s.
15. Consider the hypothetical atom pictured at right.
Determine the wavelength of the photon associated with
a transition between n = 5  n = 2. Was this an
absorption or an emission?
16. Use the same hypothetical atom at right. During an electron capture event (a free electron becomes bound to
the nucleus), the free electron becomes contained in the n = 3 orbital. What wavelength photon is emitted
during this capture?
17. Consider the orbital energy diagram
pictured for hydrogen. Of the options
pictured, what is the shortest
wavelength of light associated with
the Balmer Series?
a. What part of the spectrum is
this wavelength? If in the
visible, what color is it?
18. How many different emission
wavelengths are possible for a hydrogen electron starting at n = 7 state? Side question – does the total amount
of energy emitted by the atom as an electron transitions between n = 7 and n = 1 depend on the path it takes (is
7  1 different than 7  3  1?) Why or why not?
19. A hydrogen atom initially in its ground state absorbs a photon and ends up in the n = 3 state. What is the energy
of the absorbed photon? If the atom eventually returns to the ground state, what photon energies could the
series are both absorption and emission –for example Lyman is any transition that starts or ends in n = 1. So,
the diagram above is only half the possible transitions associated with Hydrogen, because it is missing all of the
arrows for the Lyman, Balmer, and Paschen series absorptions.
20. If light of wavelength 50 nm was incident on a hydrogen atom, what would be the kinetic energy of the
photoelectrons? Moodle: What stopping potential would be required for these electrons?
21. The photoelectric threshold wavelength of a tungsten surface is 272 nm. Calculate the maximum kinetic energy
of electrons ejected from tungsten by ultraviolet radiation of frequency 1.45 x 1015 Hz. Express your answer in
electron volts.
22. What would the minimum work function for a metal have to be for visible light (which has a range of 400 nm –
700 nm) to eject photoelectrons. Answer in eV.
23. The work function for potassium is 2.3 eV. If light having a wavelength of 250 nm hits potassium, what is the
kinetic energy of the ejected electrons in eV and what is the stopping potential in Volts. Add these two values
together. Hint: what do you notice about the two answers? Why is this the case? Think about the definition of
electron volts.
24. In a set of experiments with the hypothetical atom seen at right, you
measure the wavelengths of the photons emitted from the transitions
shown (all ending at n = 1). You also observe that it takes 17.50 eV to ionize
this atom. What is the energy of each of the energy levels (1, 2, 3, 4, and 5)
25. What wavelength of light would the atom from the previous problem emit if an electron made a transition from
n = 4 to n = 2? Answer in meters.
26. How much energy is released during a
239
94𝑃𝑢
27. How much energy is released during a
24
11𝑁𝑎
28. How much energy is released during a
15
8𝑂
alpha decay in MeV? Write the nuclear equation.
beta-minus decay in MeV? Write the nuclear equation.
beta-plus decay in MeV? Write the nuclear equation.
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