Reading Quiz

Question 1:

If you know the energy(E) of a photon of light, you know its frequency(f), wavelength(l), and momentum(p).Using the symbols E, f, l, and p along with Planck's constant (h) and the speed of light (c), relate E to f, l, and p.

Answer:

  1. f=h/E, l=c/(h/E), p=E/c.
  2. E=hƒ , E=hc/λ , E=pc
  3. E=hf=cp
  4. E=hf=hc/I=pc
  5. E=hf=hc/l
  6. E=plf
  7. E=(h*c)/I and E=h*f and E=p*c
  8. E is equal to h*f and is equal to (h*c)/lambda
  9. The Maxwell equations can be used to determine the speed of light which strongly suggests that they are waves. Young's experiment with diffraction which showed that light has properties of a wave as it bends around objects
  12. E=p*f*wavelength gives you the right units of joules.
  13. E = hf = hc/l = pc

Question 2:

Briefly describe (don't just name) two (2) pieces of experimental evidence that suggest that light can act as a particle.

Answer:

  1. The photoelectric effect explained showed that electrons emission from a metal is dependent on the frequency of individual photons, not just the intensity of the wavefunction. Compton explained through momentum (a particle concept) the wavelength of a reemitted photon from an electron that had absorbed an x-ray.
  2. 1) When light is focused on a metal plate, the electrons emitted from the plate all have the same kinetic energy. That is explained by light consisting of many particles that collide with the metal, and each electron emitted was hit by only one photon and received the same amount of energy. 2) When light strikes an electron and is reflected, the momentum of the electron and the reflected light are equal to the momentum of the light before it hit the electron. Light follows the conservation of momentum in collisions.
  3. light transfers Energy like a particals a partical theory explains lights reflections
  4. Photoelectric effect - light incident on a metal plate creates a potential difference and creates a current. Max KE is independent of the intensity, therefor light must be a particle. Compton Scattering - the photon and electron collide and the photon changes frequency.
  5. From one of Einstein's experiments we know that when light strikes a surface, the kinetic energy of the emitted electrons is independent of the intensity of the light. Light energy is quantized in small bundles called photons. The energy of light depends on the frequency of these photons. Also, Compton used the photon concept to explain the scattering of X rays by free electrons. He found that photons carry momentum.
  6. When light with a specific frequency enters a vacuum and hits a metal surface it causes electrons to be emitted which hit a second metal plate with creates a current. The second metal plate is negatively charged so only the electrons with the most energy reach it. The maximum kinetic energy of the electrons can be found by increasing the voltage. This maximum kinetic energy was found to be independent of the intensity of light. If a photon collides with an electron and scatters, the electron would absorb some energy. The scattered photon would have less energy, lower frequency, and a longer wavelength. Compton found experimentally that the photon concept was correct by measureing the new wavelength compared to the scattering angle.
  7. Light exchanges energy in quantized units of energy which is only a property of a particle. Also in Einstein's experiments of the photoelectric effect, he determined that light energy comes is bundles called photons, which act like particles.
  8. Einstein's photoelectric effect: when it is realized that the maximum kinetic energy of the electrons emited from the Cathodeis independent of intensity, it is likely that the light must be quantized in amounts Einstein called photons, which are particles. Millikan was actually the scientist who carried out this experiment and verified that the maximum kinetic energy is proporitonal to the frequency of the light by planck's constant. Compton Scattering: Compton found that the scattering of x rays caused by free electrons could be explained with the concept of photons. He simply hypothesized that the reradiation of the x ray when hitting a free electron is the same as photons scattering after a collision.
  9. the Photo electirc effect which shows that light has energy at acts as a particle when it collides with certain particles
  12. Light can act as a particle because it bounces off of things. Light can act as a particle because it can travel along a wave like a particle does.
  13. The Photoelecric Effect: bombarding a material with photons causes electrons to come out. Compton Scattering: reradiated light acts like light that has bumped into other particles and bounced out

Question 3:

Briefly describe (don't just name) one (1) piece of experimental evidence that suggests that electrons can act like waves.

Answer:

  1. The double slit experiment suggests that light acts as a wave because an interference pattern is produced just like water waves.
  2. Electrons that travel through two close slits are diffracted and interfere with each other.
  3. when light passes through a slit comprabule to the wave length then it bends, and particals can't bend spread out or like waves.
  4. Davisson/Germer - Beam of electrons diffracts through a crystal of nickle. The resultant wavelenth they found agreed with the de Broglie equation.
  5. Shooting electrons thru a slit, and they will exhibit ineterference patterns. this is a property of waves
  6. When an electron is scattered off of an object there is a certain strong scattering maximum. The angle is can be used to calculate a wavelength which agreed with the de Broglie formula.
  7. Electrons have been shown to interfere with each other just as a light wave does.
  8. The Davisson-Germer experiment bounced electrons off of a nickel crystal and observed the results collected at a detector. they saw how the electrons scattered like waves would instead of all going to the same place like a bouncing ball might. They also found that they could vary the incident electron energies and obtain diffraction patterns. Their work is all consistent with de Broglie's hypothesis.
  9. The Fizeau method is there a demo for it cause i want to see it in action. And how fast does it need to be going for it to work
  12. We saw that electrons can act like waves in that emf program online that shows the changing E field from a moving electron, showing its wave-like properties.
  13. Diffraction: electrons interfere destructively as well as constructively

Question 4:

What concept(s) or application(s) from the reading did you find interesting or intriguing? Anything you'd like to discuss further?

Answer:

  1. N/A
  2. How electrons have properties of waves.
  4. It's all pretty interesting. I'm sure we'll cover everything in class that I really want to cover ;).
  5. i thought the concept of all matter behaving like a wave was interesting.
  6. The photoelectric effect
  7. The fact the one aspect of light act like a particle and another aspect acts like a wave is very strange.
  8. I really think all of this is pretty neat. It just bugs me that there isn't a clear answer at least yet. It's kind of neat that this stuff is finally the fairly recent things in physics that people are still exploring. I liked relativity last semester and it's cool that this is tying into to those principles.
  10. Fizeau's demo
  12. I still don't understand how light can be a particle and a wave when nothing else can.
  13. What visible quantity is affected by electron waves? That's the part that confuses me the most.

Question 5:

What (if any) were the conceptual or mathematical difficulties that you had with this reading? What do we need to spend class time on?

Answer:

  1. N/A
  2. What causes particles to be diffracted when they travel though small enough openings? Does the wavelength of an electron correspond to actual oscillations, or does it correspond to some other property of an electron that is just referred to as wavelength?
  4. I'm pretty good to go with most of it.
  5. nope
  6. Nope
  7. Nope.
  8. This is probably the first time everything in the reading has been completely new to me. I get a little lost through the derivations but i think that i understand most of the concepts.
  11. Class time should be spent on why light is so weird.