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Properties of Light Is Light a Wave or a Particle?

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Bohr’s model of the hydrogen atom led to discussion about distinct spectra of light given off when it’s electron jumped from a higher to a lower energy level This discussion revolves around how to explain the behavior of light. Does it behave like a wave? A particle? Both?

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Orbitals – places on the energy levels of the atom where electrons “hang out” Ground State – the lowest energy state of an atom Excited State – the energy state of an atom when an electron absorbs a specific amount of energy to move from a lower to a higher energy level

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Wave Description of Light Electromagnetic Radiation – a form of energy that exhibits wavelike behavior as it travels through space Ex: Visible light, X rays, Ultraviolet, Radio waves, Gamma Waves Electromagnetic Spectrum – all types of energy that exhibit wavelike behavior

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Electromagnetic Spectrum

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Wavelength vs. Frequency Wavelength (λ) – the smallest distance between two repeating points on a wave λ is usually measured in meters (m), centimeters (cm) or nanometers (nm) ***NOTE: the visible light spectrum (ROYGBIV) is between 400-700 nm

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Frequency (ν) – the number of waves that pass a given point per unit of time ν is usually measured is waves per second (1/s or s -1 ) or Hertz (Hz) All electromagnetic waves travel at the speed of light (c) c = 3.0 x 10 8 m/s

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Frequency & wavelength are inversely related to one another. The mathematical equation used to describe the wavelike behavior of light is: c = λν

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Practice Problem: Calculate the frequency of light that has a wavelength of 700.0 nm. What color of light is this?

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Particle Description of Light Photoelectric Effect: electrons emitted from a metal when light shines on it. This phenomena could not be explained by the wave theory. Max Planck partially solved the issue by describing quantum.

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Photoelectric Effect When light strikes a metal it release e- some of the time. There was a threshold energy that had to be met for the metal to release an electron. Planck stated that e- had a minimum energy that had to be met before they would start emitting light: quantum.

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Quantum – light energy that is emitted or absorbed in small specific amounts Photon – a particle or packet of light carrying a specific quantum of energy German physicist, Max Planck, proposed the following mathematical relationship to determine the quantum of light given off or absorbed by electrons: E photon = hν

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E = the quantum of energy given off or absorbed by electrons, measured in joules (J) h = Planck’s constant (6.626 x 10 -34 J s) ν = frequency ( hertz or 1/s or s -1 )

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Practice problem: Determine the energy in kilojoules of a photon whose frequency is 2.98 x 10 16 s -1

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