Matter and Particles of Light: Quantum Theory

1. Matter and Particles of Light: Quantum Theory • Light (energy) and matter in motion behave both aswaves and particles • Wave-Particle Duality - Quantum Theory • Particles of light are calledphotons: E = hf = hc/l • Photons of a specific wavelength l may be absorbed or emitted by atoms in matter • Matter is made of different natural elements: lightest Hydrogen (1 proton), heaviest Uranium (92 protons) • Smallest particle of an element is atom, made up of a nucleus (protons and neutrons), and orbiting electrons • Electrons and protons attract as opposite electrical charges, NOT gravitationally like planets and Sun

2. The simplest atom: Ordinary Hydrogen Resemblance to planets orbiting the Sun is superficial ! Electrons also move both as particles and waves p – positively charged e – negatively “ One proton in the center (nucleus) and one electron in orbits of definite energy; Ordinary H has no neutrons, but ‘heavy hydrogen’ has one neutron in the nucleus

3. Absorption and emission of photons by H-atom An electron may absorb or emit light photons at specific wavelength Wavelength (n = 3  n = 2): 6562 Angstroms (RED Color) Energy of the photon must be exactly equal to the energy difference between the two ‘orbits’

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5. Continuum n= n=1 (Ground State) n=3 (2nd excited state) n=2 (1st excited state) n=4 n=5 Energy Level Diagram of 1H

6. 26 25 24 n=23 n=6 n=1 (Ground State) n=3 (2nd excited state) n=2 (1st excited state) n=4 n=5 Photons of all other energies (wavelengths) are ignored and pass on by unabsorbed.

7. 62 52 42 n=32 n=6 n=1 (Ground State) n=3 (2nd excited state) n=2 (1st excited state) n=4 n=5 Larger Jump = More Energy = Bluer Wavelength

8. Energy, Frequency, Wavelength • Light particles ‘photons’ have a unique wavelength • The more ‘energetic’ a wave, the higher its frequency, or lower its wavelength • Planck’s Law: Photon energy (‘quantum’) is E = h f = h / l • ‘h’ is the Planck’s constant • This ‘quantum’ of energy must be equal to the difference in energies between two electron orbits, for either absorption or emission by an atom

9. Mercury Spectrum of a Fluorescent Light

10. Characteristic spectra of elements Each element has a unique set of spectral lines, thus enabling its identification in the source. Observations of spectra of different elements in a source (planet, star, galaxy etc.) yields its chemical composition

11. Continuous, Absorption, and Emission spectra of a source Continuous spectrum covers wavelengths in a given range; absorption or emission spectrum consists of dark or bright lines respectively at definite wavelengths