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Lecture 2010/19/05

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wavelength Amplitude Node Electromagnetic Radiation (Light as waves) Moving Waves

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c = λν c = speed of light (3 x 10 8 m/s in a vacuum) λ = wavelength (m) ν = frequency (s -1 or Hertz, Hz)

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Electromagnetic Radiation Red light has = 700 nm. Calculate the frequency.

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Standing (stationary) Waves Has 2 or more nodes Distance between nodes is λ/2. Distance between ends has to be n(λ/2)

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a)Draw a standing wave with 1 node. What is the wavelength of this wave? b)Draw a standing wave with 3 nodes between the ends. What is the wavelength? c)If the wavelength of the standing wave is 2.5 cm, how many waves fit within the boundaries? How many nodes?

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Visible Light 1.Which color in the visible spectrum has the highest frequency? 2.Is the wavelength of x-rays longer or shorter than UV?

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The frequency of radiation used in microwave ovens is 2.45 GHz (1 gigahertz is 10 9 s -1. What is the wavelength in nm of this radiation?

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Light as particles Max Planck- Vibrations are quantized Planck’s constant E=hν = hc/λ E = energy (J) h = Planck’s constant 6.626 x 10 -34 J-s

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Photoelectric Effect

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Classical theory said that Energy of ejected electron should increase with increase in light intensity NOT OBSERVED No e - observed until light of a certain minimum E is used Number of e - ejected depends on light intensity. Light consists of particles called PHOTONS of discrete energy.

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Photoelectric Effect E electron = E light - E ejection

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Compare the energy of a mole of red light photons (λ= 700 nm) and a mole of UV photons (λ= 300 nm)

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Dual Nature of Light Both wave and particle characteristics Wave Refraction Diffraction Particle Photoelectric effect

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Diffraction Light bends as it moves through a slit or around a boundary

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Refraction Bending of light as it passes between materials of different optical density.

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Line Emission Spectrum “Excited” atoms emit light

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Line Emission Spectrum

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Balmer series Rydberg equation Balmer Series

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Atomic Spectra and Bohr 1.Any orbit should be possible and so is any energy. 2.But a charged particle moving in an electric field should emit energy. Electron would eventually run out of energy

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Bohr New theory : New theory : Quantum or Wave Mechanics e- can only exist in certain discrete orbits e- can only exist in certain discrete orbits Stationary states Stationary states e- is restricted to QUANTIZED energy states. e- is restricted to QUANTIZED energy states.

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n= principal quantum number n is an integer n with the lowest possible energy is said to be in the ground state Electrons with higher energy than ground state are said to be in an excited state

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Calculate the energies of n=1, n=2, and n=3 states of the hydrogen atom in J/atom. R = 1.097 x 10 7 m -1 h = 6.626 x 10 -34 J-s c = 2.998 x 10 8 m/s

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Moving between energy levels

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Calculate the wavelength of the green light of excited H atoms.

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