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Chapter 12

Chapter 12. Sections 12.6 to 12.10 Wavelength. Light. The study of light led to the development of the quantum mechanical model. Light is a kind of electromagnetic radiation.

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Chapter 12

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  1. Chapter 12 Sections 12.6 to 12.10 Wavelength

  2. Light • The study of light led to the development of the quantum mechanical model. • Light is a kind of electromagnetic radiation. • In Wave Model, Light is considered to consist of electromagnetic waves that travel in a vacuum @ speed of 3.00 x 108 m/s • Electromagnetic radiation includes many kinds of waves • All move at 3.00 x 108 m/s ( c)

  3. ER • Electromagnetic Radiation – forms of energy that exhibits wavelight behavior as it travels thru space • Examples of Electromagnetic Radiation: • Radio Wvaes • Microwaves • Visible Light • Infrared • Ultraviolet • X-Rays • Gamma Rays • ER has measurable wave properties of wavelength and frequency

  4. Crest Wavelength Amplitude Trough Parts of a wave Orgin

  5. Parts of Wave • Orgin - 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 l Greek letter lambda.

  6. Frequency • The number of waves that pass a given point per second. • Units are cycles/sec or hertz (hz) • Abbreviated n the Greek letter nu c = ln

  7. Frequency and wavelength • Are inversely related • As one goes up the other goes down. • Different frequencies of light is different colors of light. • There is a wide variety of frequencies • The whole range is called a spectrum

  8. Frequency and Wavelength are mathematically related, they are inversely related • The relationship is shown by the following equation: • c = ln • c= speed of light • l= Wavelength

  9. Long Wavelength = Low Energy Low Frequency • Short Wavelength = High Energy High Frequency

  10. High energy Low energy Low Frequency High Frequency X-Rays Radiowaves Microwaves Ultra-violet GammaRays Infrared . Long Wavelength Short Wavelength Visible Light

  11. Calculating Light • What is the wavelength if light with a frequency of 5.89 x 105 Hz? • What is the frequency of blue light with a wavelength of 484 nm?

  12. H.W. Questions • List 5 examples of E.R. • What is the speed of all forms of E.R. in a vacuum • Relate Frequency and Wavelength • The speed of light is 3.00 x 108 m/s and the frequency is 7.500 x 1012 Hz. Calculate the Wavelength of E.R. • Determine the frequency of light w/ a wavelength of 4.257 x 10-7 cm.

  13. Atomic Spectrum How color tells us about atoms

  14. Spectrum • Spectrum- Range of wavelengths of E.R., wavelengths of visible light are separated when a beam of white light passes thru a prism • Example of Spectrum • Rainbow (also a phenomenon) • Each droplet of water acts as a prism to produce a spectrum • Each color blends into the next color.

  15. Colors of the Spectrum • Colors of the Spectrum: • Red (Longest Wavelength & Lowest Frequency) • Orange • Yellow • Green • Blue • Indigo • Violet (Shortest Wavelength & Highest Frequency) • These colors are known as the visible part of the spectrum

  16. Prism • White light is made up of all the colors of the visible spectrum. • Passing it through a prism separates it.

  17. Diffraction • When light passes through, or reflects off, a series of thinly spaced line, it creates a rainbow effect • because the waves interfere with each other.

  18. A wave moves toward a slit.

  19. Comes out as a curve

  20. with two holes

  21. Two Curves with two holes

  22. Two Curves with two holes Interfere with each other

  23. Two Curves with two holes Interfere with each other crests add up

  24. Several waves

  25. Several waves Several Curves

  26. Several waves Several waves Several Curves Interference Pattern

  27. Spectroscopic analysis of the visible spectrum… …produces all of the colors in a continuous spectrum

  28. If the light is not white • By heating a gas with electricity we can get it to give off colors. • Passing this light through a prism does something different. • More on that Later

  29. Quantum Concept • Laws of Physics state no limits on how much or how little energy can be gained or lost. • Classic physics assumed atoms and molecules could emit any arbitrary amount of radiant energy. • Does not explain the Emission Spectrum of Atoms

  30. Max Plank • Tried to explain why the body changed colors as it heated. • He could only explain the change if assumed energy of the body changes in small discrete units (brick by brick). • Plank showed mathematically that the amount of radiant energy, absorbed or emitted by a body is proportional to the frequency of radiation

  31. Plank’s Quantum Concept • Plank went against classic physics • Stated: atoms and molecules could emit energy only is discrete quantities, like small packages or bundles • Quantum- smallest quantity of energy that can be emitted in the form of E.R. • The energy of a single quantum is given by E = hν

  32. Planck’s Quantum Theory Cont. • According to theory, energy is always emitted in multiples of hν. • Example: 2hv, 3hv, ect….. • Never in 1.67hv and so on • Could not explain why energies are fixed but explained the emission of solids over the entire range or wavelengths.

  33. Energy and frequency • E = h x n • E is the energy of the photon • n is the frequency • h is Planck’s constant • h = 6.6262 x 10 -34 Joules sec. • joule is the metric unit of Energy

  34. Examples • What is the wavelength of blue light with a frequency of 8.3 x 1015 hz? • What is the frequency of red light with a wavelength of 4.2 x 10-5 m? • What is the energy of a photon of each of the above?

  35. Solving for photons • Calculate the energy of: • A) photon with a wavelength 5.00 x 104 nm (infrared region) • B) photon with a wavelength of 5.00 x 102 nm (X ray region)

  36. Einstein and Photoelectric Effect • 5 years later Einstein used Planck’s theory to derive the Photoelectric Effect • Photoelectric Effect- a phenomenon in which electrons are ejected from the surface of certain metals exposed to light of at least a certain minimum frequency • Threshold Frequency

  37. Photoelectric Effect • Number of electrons ejected was proportional to the intensity (brightness) of the light, but the energies of the electrons were not. • Below the threshold frequency no electrons were ejected no matter how intense the light. • Could not be explained by the wave theory of light.

  38. Photoelectric cont. • Einstein proposed: • That a beam of light is really a stream of particles. • Particles of light are now called Photons. • Used Planck’s equation to determine: • Electrons are held in a metal surface by attractive forces, and removing them from the metal requires light of a sufficiently high frequency ( which corresponds to sufficiently high energy) to break them free.

  39. Shining a beam of light onto a metal surface can be though as shooting a beam of particles/photons at the metal atoms. • Frequency of photons = binding energy, then light will have just enough energy to knock them free • What if the frequency is higher?

  40. Photoelectric cont. • If frequency is stronger they will acquire some K.E. and be knocked loose. • KE = hv – BE • Shows more energetic the photon, greater the K.E. of the ejected electron.

  41. Wave-Particle Duality JJ Thomson won the Nobel prize for describing the electron as a particle. His son, George Thomson won the Nobel prize for describing the wave-like nature of the electron. The electron is a particle! The electron is an energy wave!

  42. Light is a Particle • Energy is quantized. • Light is energy • Light must be quantized • These smallest pieces of light are called photons. • Energy and frequency are directly related.

  43. Energy Change • The size of an emitted or absorbed Quantum depends on the size of the energy change. • Ex. • Small energy change involves emission or absorption of low frequency radiation. • Large energy change involves emission or absorption of high frequency radiation.

  44. The Math in Chapter 12 • Only 2 equations • c = ln • E = hn • Plug and chug.

  45. An explanation of Atomic Spectra

  46. Atomic Spectrum • Each element gives off its own characteristic colors. • Can be used to identify the atom. • How we know what stars are made of.

  47. Atomic Emission Spectrum • Atomic Emission Spectrum- it passes light emitted by an element thru a prism • Atoms first absorb energy and then lose energy as they emit light • Each line in the emission spectrum corresponds to one exact frequency of light being given off or emitted by an atom. • The light emitted by an electron moving from a higher to lower energy level has a frequency directly proportional to the energy change of the electron • Therefore each line corresponds to one exact amount of energy being emitted. • Emission Spectrum of each element is unique to that element. • The emission spectrum is obtained by an instrument called Emission Spectrograph

  48. These are called discontinuous spectra • Or line spectra • unique to each element. • These are emission spectra • The light is emitted given off.

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