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Lecture 2010/19/05

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

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)

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

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

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?

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?

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?

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

Photoelectric Effect

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.

Photoelectric Effect E electron = E light - E ejection

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

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

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

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

Line Emission Spectrum “Excited” atoms emit light

Line Emission Spectrum

Balmer series Rydberg equation Balmer Series

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

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.

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

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

Moving between energy levels

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

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