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Published byGenesis Greenhouse Modified about 1 year ago

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Electromagnetic Radiation

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Definitions Electromagnetic Radiation is energy with wavelike characteristics Moves at a speed of 3.0 x 10 8 m/s

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Wavelength (λ) is the distance between identical points on successive waves – meters Frequency (ν) is the waves passing per second – unit is the Hertz (Hz)

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Each type of radiant energy has its own characteristic frequency and wavelength Short wavelength then high frequency Long wavelength then low frequency

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EM Equation c = νλ Yellow light given off by a sodium lamp has a wavelength of 589 nm. What is its frequency? Radiation of high frequency has more energy than that with low frequency

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Quantum Theory Rules that govern the gain and loss of energy from an object

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Max Planck Said energy comes in “chunks” called quantum Energy of the quantum depends on the frequency ΔE = hν Planck’s constant (h) is 6.63x10 -34 J-s

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Energy Lost or Gained Total energy lost or gained is a multiple of the quantum ΔE tot = nhν n – number of quantum

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Example 1 Calculate the smallest increment of energy that an object can absorb from yellow light with a wavelength of 589 nm.

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Example 2 A laser emits light pulses of frequency of 4.69x10 14 Hz and deposits 1.3x10 -2 J on energy each pulse. How many quantum of energy is this?

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Photoelectric Effect Light shining on certain metals emit electrons A minimum frequency of light is necessary Albert Einstein explained this effect with quantum theory

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Quantum of light is called a photon E photon = hν When photons are absorbed by a metal, energy is transferred to the electrons If sufficient energy, the electron can overcome the attractive forces holding it in the atom and escape

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Amount of energy depends on the freq. If freq is too low, not enough energy for electron to escape If freq is higher than what is needed for electron to escape, the extra energy is converted into kinetic energy making the electron move faster

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Total energy of photon = energy required to free electron + kinetic energy hν = E B + E K E B : called “binding energy” or “work function”

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Example Potassium metal must absorb a wavelength of 540 nm (green) or shorter in order to emit an electron. What will be the kinetic energy of an electron if it absorbs 380 nm (UV) light?

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