Section 3 review
Is light a wave or a particle? This question confounded physicists for many centuries. In the 19th century, physicists were fairly certain that light was a wave, because they knew from Young’s double slit experiment that light can diffract and then interfere with itself. But the photoelectric effect, discovered at the end of the century, could not be explained by the wave model. Einstein soon resolved the problem of the photoelectric effect by explaining that light acts as little particles called photons, based on Planck’s equation for their energy, E = hf. While light can exhibit wave properties, at the atomic level light often exhibits properties of a particle. Quantum physics is the study of phenomena at the atomic and nuclear level where many physical properties are quantized, or only come in discrete values. An early success of the quantum theory was in explaining the photoelectric effect through quantized energy of photons. Read the text aloud
polarization, diffraction pattern, spectrograph, photon, Planck’s constant, quantum physics, photoelectric effect, threshold frequency, work function, detector, pixel, optical fibers

E=hf
E k =hf W 0

Review problems and questions

  1. Describe evidence that light behaves like a particle and evidence that light behaves like a wave. Read the text aloud Show
    1. What is the energy of a photon of light that has a wavelength of λ = 2 μm?
    2. What is the energy of a photon of light that has λ = 0.5 μm?
    3. What part of the electromagnetic spectrum does each photon correspond to? Read the text aloud Show
  1. Explain how to find the maximum kinetic energy of an electron ejected from a material via the photoelectric effect. Read the text aloud Show
  1. A researcher was designing an experiment to demonstrate the photoelectric effect using the metal nickel, which has a work function of 5.15 eV. The electron volt, or eV, is a more convenient way to express photon energies than using joules. 1 eV = 1.602×10−19 J.
    1. Explain what is meant physically by the work function.
    2. Calculate the threshold frequency for nickel. To what region of the electromagnetic spectrum does this correspond?
    3. Will light at an energy of 7.35 eV result in the production of photoelectrons from nickel? In what region of the electromagnetic spectrum is this light?
    4. Will infrared light at a wavelength of 1 μm produce photoelectrons?
    5. Will violet light (400 nm) produce photoelectrons?
    6. Will ultraviolet light at 200 nm produce photoelectrons? Read the text aloud Show
  1. Many forms of modern communication use wave pulses of infrared and visible light to transfer digital information. Describe why infrared (IR) or visible light, rather than radio frequencies, is used for transmitting the signals needed for high-definition video. Show

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