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Light and the Quantum Mechanical Model of the Atom Light • The study of light led to the development of the quantum mechanical model. • Light is a kind of electromagnetic radiation. • Electromagnetic radiation includes many kinds of waves • All move at 3.00x108 m/s or 3.00x1010 cm/s (abbreviated: c) Parts of a wave Crest Wavelength Amplitude Origin Trough Parts of Wave • • • • Origin - the base line of the energy. Crest - high point on a wave Trough - Low point on a wave Amplitude - distance from origin to crest • Wavelength - distance from crest to crest • Wavelength - is abbreviated letter lambda) l (Greek Frequency • The number of waves that pass a given point per second. • Units are cycles/sec or hertz (Hz) • Abbreviated nu) n (the Greek letter c = ln Frequency and Wavelength • Are inversely related • As one goes up the other goes down. • Different frequencies of light are different colors of light. • There is a wide variety of frequencies • The whole range is called a spectrum Electromagnetic Spectrum High Low energy energy Radio Micro Infrared Ultra- XGamma waves waves violet Rays Rays Low High Frequency Frequency Long Short Wavelength Wavelength Visible Light Energy and Frequency • • • • • E = (h)(n) E is the energy of the photon n is the frequency h is Planck’s constant h = 6.626 x 10 -34 Joules*sec. The Math You WILL need to Know for this Chapter Only 2 equations: • c = ln • E = hn • c is always 3.00 x 108 m/s • h is always 6.626 x 10-34 J•s Examples • What is the frequency of red light with a wavelength of 4.2 x 10-5 cm? • What is the wavelength of The River 105.9, which broadcasts at a frequency of 640 kHz? • What is the energy of a photon of each of the above? Atomic Spectra How light & color tells us about atoms White Light • It is made up of all the colors of the visible spectrum. • Passing it through a prism separates it. • This is called a continuous spectrum • because you see all the colors of the visible spectrum blurred together (ROYGBIV) If the light is not white • By heating or electrifying a sample of atoms scientists can get it to give off colors. • Passing this light through a prism does something different • You get colors of light that relate to specific frequencies and therefore specific energies • This is called … Atomic Emission Spectrum or Line Spectrum • Each element gives off a unique set of colors. • This info can be used to help identify atoms in substances • Ex: stars http://jersey.uoregon.edu/elements/Elements.html An explanation of Atomic Spectra How it all begins… • In the atom all electrons start in the lowest energy level they are normally found in. • This is called the ground state. Hydrogen atom • Let’s take a closer look at a hydrogen atom Changing the energy • Adding energy can move the electron up to higher energy levels • The more energy added, the more energy levels the e- moves • As the electron falls back to its original ground state it gives the absorbed energy back as light we can see • The further the e- fall, the more energy, and the higher the frequency of light. Max Planck • 1900 – from Germany • Tried to explain why metals changed color the way they did when they are heated • They only show certain colors, not all the colors in the visible spectrum. WHY? • The energy is changed in a specific amount called a quantum Light as a Particle • Energy is quantized. • These smallest pieces of light are called photons. So… What is light then?? • Light is a wave we can measure its wave length and determine its frequency and it behaves as a wave • Light is also a particle it comes in chunks called photons • Albert Einstein came up with this idea Photoelectric Effect • Metals eject electrons when light of a specific frequency (called a threshold frequency) shines on them. • The light has to have enough energy and if the light is the wrong frequency then it won’t work at all (no matter how bright the light) • Uses of this: solar cells, camera flash, & automatic doors • FYI: Explaining this effect is how Einstein won his Nobel Prize More obvious with the very small • To measure where a electron is, we use light. • But the light moves the electron • And hitting the electron changes the frequency of the light. Matter is also a Wave • BUT… • This does not apply to objects bigger than an atom b/c their wavelengths are too small • Ex: A baseball has a wavelength of about 10-32 m when moving 30 m/s • An electron at the same speed has a wavelength of 10-3 cm • This length is big enough to measure. • We called these “matter waves” The physics of the very small • Quantum mechanics explains how the very small behaves. • Quantum mechanics is based on probability because Heisenberg Uncertainty Principle • It is impossible to know exactly the speed and position of a particle. • The better we know one, the less we know the other. • The act of measuring changes the properties. Before Photon Moving Electron After Photon changes wavelength Electron changes velocity