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Section Summary

PhysicsSection Summary

21.1 Planck and Quantum Nature of Light

  • A blackbody will radiate energy across all wavelengths of the electromagnetic spectrum.
  • Radiation of a blackbody will peak at a particular wavelength, dependent on the temperature of the blackbody.
  • Analysis of blackbody radiation led to the field of quantum mechanics, which states that radiated energy can only exist in discrete quantum states.

21.2 Einstein and the Photoelectric Effect

  • The photoelectric effect is the process in which EM radiation ejects electrons from a material.
  • Einstein proposed photons to be quanta of EM radiation having energy E=hf, E=hf, where f is the frequency of the radiation.
  • All EM radiation is composed of photons. As Einstein explained, all characteristics of the photoelectric effect are due to the interaction of individual photons with individual electrons.
  • The maximum kinetic energy KEe of ejected electrons (photoelectrons) is given by K E e =hfBE, K E e =hfBE, where hf is the photon energy and BE is the binding energy (or work function) of the electron in the particular material.

21.3 The Dual Nature of Light

  • Compton scattering provided evidence that photon-electron interactions abide by the principles of conservation of momentum and conservation of energy.
  • The momentum of individual photons, quantified by p= h λ p= h λ , can be used to explain observations of comets and may lead to future space technologies.
  • Electromagnetic waves and matter have both wave-like and particle-like properties. This phenomenon is defined as particle-wave duality.
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