Unit 6 Light

Quantum Theory Max Planck (1900) Observed - emission of light from hot objects Concluded - energy is emitted in small, specific amounts (quanta) Quantum - minimum amount of energy change In 1900, Max Planck explained the “ultraviolet catastrophe” by assuming that the energy of electromagnetic waves is quantized rather than continuous—energy could be gained or lost only in integral multiples of some smallest unit of energy, a quantum. • Classical physics had assumed that energy increased or decreased in a smooth, continuous manner. • Planck postulated that the energy of a particular quantum of radiant energy could be described by the equation E = h, where h is the Planck’s constant and is equal to 6.626 x 10-34 joule•second (J•s). • As the frequency of electromagnetic radiation increases, the magnitude of the associated quantum of radiant energy increases. Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem 2

Light Particles photons (discrete bundles of energy) Light is emitted when electrons return to ground state from an excited state Waves Frequency, wavelength Wave-Particle duality 3

Formation of Light e e e e e e e e e e e e + Ground State Excited Ion is formed Li  e + Li1+ Ground State Electron Returns to Ground State Excited State e Lithium Atom Light is given off n = 7 n = 6 e e hv n = 5 n = 4 The ground state is the low-energy state where you can initially find electrons. When energy is added to the atom, the electrons may use this energy to jump into a higher-energy state called the excited state. The energy of an electron is determined by its average distance from the nucleus. Each atomic orbital with a given set of quantum numbers has a particular energy associated with it, the orbital energy. In atoms or ions that contain only a single electron, all orbitals with the same value of n have the same energy (they are degenerate). Energies of the principal shells increase smoothly as n increases. An atom or ion with the electron(s) in the lowest-energy orbital(s) is said to be in the ground state; an atom or ion in which one or more electrons occupy higher-energy orbitals is said to be in the excited state. n = 3 e n = 2 e e e e e e e e n = 1 Nucleus Nucleus Nucleus Nucleus + 4

An Excited Lithium Atom Excited Li atom Energy Photon of red light emitted Li atom in lower energy state Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 326 5

Heisenberg Uncertainty Principle It is impossible to simultaneously know the exact velocity and position of any object. In order to detect any object’s position (either by eye or with a machine), it has to be struck by light. To see an object, it has to be struck by visible light. For night-vision goggles, it has to be struck by IR light. As soon as it is struck by photons, the e- move, thereby changing its position and velocity. This hardly matters for large objects, but it does for atoms & e-.

Electromagnetic Spectrum Light is a form of energy. Energy travels in waves. Waves have two components. Amplitude (distance from center of wave to peak) Wavelength (distance from peak to peak or trough to trough)

Visible Spectrum of Light Waves 1/33,000” long Waves 1/70,000” long Red Orange Yellow Green Blue Indigo Violet PRISM Slit Ray of White Light All light is bent passing through a prism; violet is bent most and red least. A beam of sunlight produces a continuous band of rainbow colors showing that light is a mixture of colors. 9

The Electromagnetic Spectrum GH ENERGY LOW ENE RGY Decreasing wavelength Increasing frequency Increasing photon energy AM radio Short wave radio Television FM Radar Microwave Radio Waves V i s b l e L g h t Gamma Rays UV Rays “The Electromagnetic Spectrum”   Description: This slide depicts the electromagnetic spectrum from gamma rays through radio waves. Basic Concepts ·         All forms of electromagnetic radiation are not identical ·         All forms of electromagnetic radiation travel at the same speed in a vacuum (the speed of light, c = 3.00 x 108 m/sec). ·         Wavelength and frequency are inversely proportional for a wave traveling at a constant speed. ·         Energy and frequency are directly proportional for electromagnetic waves traveling at the speed of light. Teaching Suggestions Use this transparency to review the relationship of visible light to other types of radiation. Explain that all of the rays and waves shown are types of electromagnetic radiation. Point out that they differ essentially from each other only in energy level, wavelength, and frequency. Try the analogy of an ocean wave to help students understand electromagnetic waves. Question 6 can be used to assess the students understanding of wave velocity, wavelength, and frequency. Questions: List the ways in which visible light is different from the other types of radiation shown in the diagram. List the ways in which all of the types of radiation shown in the diagram are similar. You are told that sound waves cannot travel in a vacuum. Are sound waves a types of electromagnetic radiation? Explain your logic. Radio waves can go around an obstruction if the obstruction is smaller than the radio wave’s wavelength. What would you expect to happen if visible light were beamed at a thin wire 2 x 10-5 centimeter thick? Explain your answer. For electromagnetic waves traveling at the speed of light, the wavelength is inversely proportional to frequency, as expressed by the equation c = fl, where c = speed of light in vacuum (3.00 x 108 meters/second), f = frequency, and l= wavelength. Using this equation, calculate the frequency of a 3-meter radio wave traveling at the speed of light. Compare your answer with the diagram. Suppose that at a particular beach the ocean waves are traveling at a speed of 2 meters/second. If you know that the distance between waves is 10 meters, can you calculate how often they hit the shore? Explain your answer. For electromagnetic waves traveling at the speed of light, the energy of a single photon is expressed by the equation E = hf, where E = energy, f = frequency, and h = Planck’s constant, 6.6 x 10-33 joules/hertz. Which has more energy, a photon of visible light or a photon of radar, if both traveling at the speed of light? Do you think you can calculate the energy of an ocean wave using this energy equation? Explain your answer. infrared X- Rays R O Y G B I V Red Orange Yellow Green Blue Indigo Violet 10

Electromagnetic Spectrum Waves can have different wavelengths Wavelength Energy Frequency

Flame Emission Spectra Photographs of flame tests of burning wooden splints soaked in different salts. This technique is called emission spectroscopy. methane gas wooden splint sodium ion calcium ion copper ion strontium ion 12

Fireworks 13

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. 14

Common chemicals used in the manufacture of fireworks Oxidizers Fuels (Reductants) Special Effects Ammonium perchlorate aluminum Blue flame: copper carbonate, copper sulfate, or copper oxide Barium chlorate antimony sulfide Red flame: strontium nitrate or strontium carbonate Barium nitrate charcoal White flame: magnesium or aluminum Potassium chlorate magnesium Yellow flame: sodium oxalate or cryolite (Na3AlF6) Potassium nitrate sulfur Green flame: barium nitrate or barium chlorate Potassium perchlorate titanium White smoke: potassium nitrate plus sulfur Strontium nitrate Colored smoke: potassium chlorate and sulfur, plus organic dye Whistling noise: potassium benzoate or sodium salicylate White sparks: aluminum, magnesium, or titanium Gold sparks: iron filings or charcoal Almost any combination of an oxidizer and a fuel may be used along with the compounds needed to produce a desired special effect. Copyright © 2007 Pearson Benjamin Cummings. All rights reserved. 15