12.8 Light as Particles: The Photoelectric Effect Einstein expanded on Newton and Planck’s idea of light as particles Einstein expanded on Newton and Planck’s idea of light as particles Einstein stated that light could be described as quanta of energy that behave as particles Einstein stated that light could be described as quanta of energy that behave as particles These light quanta are called photons These light quanta are called photons The energy of a photon is quantized by the equation: The energy of a photon is quantized by the equation: E = hν

12.8 The Photoelectric Effect In the photoelectric effect, electrons called photoelectrons are ejected by metals when light shines on them. In the photoelectric effect, electrons called photoelectrons are ejected by metals when light shines on them. Not just any type of light will do; light must be above threshold frequency to cause photoelectrons to be ejected Not just any type of light will do; light must be above threshold frequency to cause photoelectrons to be ejected The photoelectric effect was very influential in understanding the wave-particle duality of light The photoelectric effect was very influential in understanding the wave-particle duality of light The photoelectric effect is related to the photovoltaic effect which utilizes light to generate electricity The photoelectric effect is related to the photovoltaic effect which utilizes light to generate electricity

12.8 The Photoelectric Effect Only light that is of a specific frequency will cause photoelectrons to be ejected Only light that is of a specific frequency will cause photoelectrons to be ejected Increasing the intensity (amount) of light causes more electrons to be ejected Increasing the intensity (amount) of light causes more electrons to be ejected Increasing the frequency will cause the ejected electrons to move faster Increasing the frequency will cause the ejected electrons to move faster

12.8 Example 3 Calculate the energy, in joules, of a quantum of radiant energy (the energy of a photon) whose frequency is 5.00 x s -1. Calculate the energy, in joules, of a quantum of radiant energy (the energy of a photon) whose frequency is 5.00 x s -1.Solution: E = hν E = ( x J∙s)(5.00 x s -1 ) E = 3.31 x J

12.9 An Explanation of Atomic Spectra Electrons in their lowest energy level are said to be in their ground state. Electrons in their lowest energy level are said to be in their ground state. The energy levels are quantized so it takes quantum energy, h x v to raise the electrons from ground state to excited state The energy levels are quantized so it takes quantum energy, h x v to raise the electrons from ground state to excited state This same amount of energy is emitted when the electron drops back down and the electron emits light (law of conservation of energy) This same amount of energy is emitted when the electron drops back down and the electron emits light (law of conservation of energy)

12.9 An Explanation of Atomic Spectra Example: Hydrogen has one electron; for its ground state, quantum number (n)=1 Example: Hydrogen has one electron; for its ground state, quantum number (n)=1 The electron can be excited so that n = 2, 3, 4, or 5, and so forth. The electron can be excited so that n = 2, 3, 4, or 5, and so forth. What energy level the electron is at and to what energy level it drops to determines the emission spectrum. What energy level the electron is at and to what energy level it drops to determines the emission spectrum.Hydrogen: