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  1. Fiat LuxReview of the Electro-Magnetic Spectrum Kate Martin McCrone Associates, Inc.

  2. Outline • Historical understanding of light • Properties of light • Light beyond the visible region • Electromagnetic spectrum • Spectroscopy • Regions of the spectrum • Radio • Microwave • Infrared • Visible • Ultraviolet • X-ray • γ-ray

  3. Google Hits • Spectroscopy 18,500,000 • Spectroscopy and materials 10,200,000 • Spectroscopy and chemistry 9,370,000 • Spectroscopy and biology 3,320,000 • Spectroscopy and polymers 2,380,000 • Spectroscopy and food 2,060,000 • Spectroscopy and astronomy 1,510,000

  4. What is the electromagnetic spectrum? • Broadcast signal • Dental x-ray • Rainbow • Supernova burst • Sunburn • Fireworks • Medical imaging

  5.                                                                                                    <> The Electromagnetic Spectrum high frequency low frequency short wavelength long wavelength

  6. Electromagnetic Spectrum low frequency high frequency long wavelength short wavelength Courtesy of the Advanced Light Source, Berkeley Lab

  7. Transmittance Through Atmosphere

  8. http://hyperphysics.phy-astr.gsu.edu/hbase/mod4.html#c1

  9. Visible Light

  10. Aristotle (~350 BCE) • “Thus, pure light such as that from the sun, has no color, but is made colored by its degradation when interacting with objects having specific properties which then produce color.” • This was the common view for about 20 centuries.

  11. Isaac Newton (1642 – 1727) Portrait of Isaac Newton in 1689 (aged 46 years)

  12. What Does a Prism Do? • Prisms already a well-known toy in Newton’s day • Color was believed by many to be a homogeneous mixture of light and dark and that prisms acted to modify the light (that is, prisms added the color) • Newton demonstrated that prisms did not modify light; they acted to separate light into its constituent colors; thus, light is heterogeneous

  13. “Experimentum Crucis” ~1666 White light was first decomposed into its constituent colors, then recombined back into white light, then separated again. Light was further deflected by subsequent prisms, but the color was not modified. Thus, white light is a heterogeneous mixture of differently refrangible colors.

  14. Not Everyone Agreed 1790 - Johann Wolfgang von Goethe Light is “the simplest most undivided most homogenous being that we know. Confronting it is the darkness“ “Yellow is a light which has been dampened by darkness” “Blue is a darkness weakened by the light”

  15. Nature of Light (Early Ideas) • Corpuscle (particle) nature (Newton) • Red corpuscles the largest (the least deflected) • Violet corpuscles the smallest (greatest deflection) • Wave nature (Christian Huygens) • “Movement within a fine medium” • Did not intially recognize variation in wavelengths

  16. Wave Nature of Light c λ=wavelength y= amplitude c= speed of light in a vacuum f = frequency (cycles per second)

  17. Wave Nature of Light

  18. Properties of Light • Refraction • Light passing from one medium into another changes direction • With a prism, blue light shows a greater change in direction than red light • Diffraction • Apparent bending of light around small obstacles • Reflection • Change of direction at an interface such that light returns back to the original medium • Specular (mirror-like) reflectance: angle of incidence equals angle of reflectance • Scattering • Diffuse reflectance, internal reflectance • Absorption • Transmittance

  19. Is the straw broken? No, we see the straw through different media, with different refractive indices Refraction

  20. Refraction Light bends as it goes from air into the glass balls, and then again as it goes back into air

  21. n = Refractive Index Velocity of light in a vacuum Velocity of light in a medium

  22. Refractive Index • Materials may have one or more refractive indices • Air 1.000293 • Water 1.3330 • Borosilicate Glass 1.470 • Diamond 2.419 • Calcite (CaCO3) 1.49, 1.66 • Materials with two or more refractive indices are birefringent a crystal of calcite

  23. Refractive Index • Identification • Solids – Light Microscope • Minerals • Glasses • Powders • Liquids – Refractometer, HPLC detector • Water • Organics • Solutions • Gases • Purity • Concentration • Calculate focusing power of lenses • Calculate dispersive power of prisms

  24. Diffraction • Hologram on a credit card (diffraction grating) • Rainbow pattern on a CD or DVD • Bird feathers a spider web’s color is partially due to diffraction

  25. Diffraction • Diffraction is the apparent bending that occurs for a wave when it encounters an obstacle, or the spreading out of a wave past small openings circular aperture (Airy disk) slit which is 4 wavelengths long

  26. Diffraction • X-ray diffraction • Location and distances between atoms in a crystal • Holography • Mixing of a laser beam and unfocused diffraction pattern of some object • Diffraction grating as a dispersive element • monochromator

  27. Reflection • Seeing yourself in a mirror • Reflection of sun in lake waters • Mirage in the desert

  28. Specular Reflectance

  29. Reflection • Light, sound and water waves undergo reflection • Reflection of VHF is important for radio transmission and radar • Hard x-rays and gamma rays can be reflected with grazing angle mirrors

  30. Reflectance Double reflection – sun reflected in water and again in the paddle Caravaggio, “Narcissus”

  31. Diffuse Reflectance

  32. Scattering (Diffuse) • Light, sound, particles can all be scattered • Defined as a deviation from a straight trajectory by one or more localized non-uniformities • Scattering centers include bubbles, particles, irregular surfaces (sandpaper), droplets, defects in crystals, surface roughness, textured (fibrous) clothing, etc. • Areas that use scattering measurements include medical imaging, radar, inspection of silicon wafers, process monitoring (polymer synthesis), computer imagery

  33. Scattering • Elastic Scatter (wavelength is unchanged upon scattering) • Mie: • Scattering center ~ wavelength or larger • Shape-dependent • Wavelength independent • White clouds (water droplets are larger than air molecules) • Rayleigh: • Scattering centers small compared to wavelength (<1/10) • Wavelength dependent I ∞ 1/Λ4 • Scattering at 400 nm is 9.4 x that at 700 nm • Blue sky (air molecules are small compared to visible light) • Inelastic scatter (wavelength is changed upon scattering) • Raman, Compton, inelastic x-ray

  34. Absorption / Transmission • Light absorbed by a material may impart some of its energy (for example, as heat) • Absorption/transmission is selective: • Dark materials absorb most/all visible wavelengths • Glass transmits visible light, but absorbs UV • Chlorophyll absorbs blue and red, and reflects green • Absorbance can be used to quantify a material, as in the Beer-Lambert Law A = ελ x b x c • A = absorbance • ελ = wavelength-dependent absorptivity coefficient • b = pathlength • c = concentration

  35. Adventures of a Beam of Light Emission (fluorescence) Light IN internal reflectance Absorption specular reflectance Light OUT (transmitted) diffuse reflectance Semi-transparent material

  36. When the sun is near the horizon, its rays pass through more atmospheric particulate than when overhead (longer pathlength) Short wavelengths are efficiently scattered away, leaving red light to reach your eye Why is the Sunset Red?

  37. The Scream – Edvard Munch

  38. Discovery of Non-Visible Light - I Sir Frederick William Herschel 1738 - 1822

  39. Sir Frederick William Herschel • Astronomer, discovered the planet Uranus (1781) • Noticed that differently colored filters appeared to pass different amounts of heat from sunlight • 1800: Measurement of color temperatures using thermometers and a glass prism to separate light into component colors • Temperature increased from violet to red • Highest temperature just outside the red (calorific rays) • Calorific rays underwent reflection, refraction, absorption, transmittance like visible light

  40. Discovery of Infrared Crude temperature measurements of light

  41. Thought that nature had “polarity” and if radiation existed outside one end of the visible light spectrum, it must exist outside the other end. 1801: Glass prism and paper soaked in AgNO3 – blue light darkens AgNO3 more/faster than red. Just beyond the violet – it darkens even more! “Chemical rays” now known as ultraviolet Independently discovered in 1802 by William Hyde Wollaston Discovery of Non-Visible Light- II Johann Wilhelm Ritter born in 1776 in Samitz, Silesia

  42. Electro + Magnetic • Oersted (1820): • Electrical and magnetic fields first associated in 1820 • Electrical current through a wire could produce deflection in a compass needle • Ampere: • Two wires conducting electrical current could repel each other • Faraday (1847): • Proposed that light was a high frequency electromagnetic vibration • Maxwell (1864): • Mathematically explained link between electricity and magnetism • Determined that visible light was a form of EM radiation

  43. The Photon • The photon (from Greek "phos", meaning light) is the quantum of the electromagnetic field (smallest unit). The term photon was coined by Gilbert Lewis in 1926. • In some respects a photon acts as a particle, for instance when registered by the light -sensitive device in a camera. • In other respects, a photon acts like a wave, as when passing through the optics in a camera.

  44. Electromagnetic Spectrum E = hc/λ f = c/λ E = Energy of a photon h = Planck’s constant, 6.626 x 10-34 J•sec c = speed of light in a vacuum, 3.0 x 108 m/sec λ = wavelength (m) f = frequency (Hz)

  45. Radio Microwave Infrared Visible Ultraviolet X-ray γ-ray < 3x109 Hz 3x109 – 3x1011 Hz 3x1011 – 4x1014 Hz 4x1014 – 7.5x1014 Hz 7.5x1014 – 3x1016 Hz 3x1016 – 3x1019 Hz > 3x1019 Hz Units RegionFrequencyWavelength > 10 cm 10 – 0.1 cm 1000 – 0.7 µm 700 – 400 nm 400 – 10 nm 10 – 0.01 nm < 0.01 nm

  46. Spectroscopy • Interaction of light with matter to study certain chemical and physical properties of matter • Now generally includes use of particles (mass) or alternating field in addition to light • ~130 types of spectroscopy • Identification • Structure • Quantitation • Physical properties

  47. Interaction of EM with Matter http://hyperphysics.phy-astr.gsu.edu/hbase/mod3.html#c1

  48. Interactions with Matter Nuclear spin Molecular rotations Molecular vibrations Outer shell electrons Molecular dissociation Inner shell electrons Nuclear transitions Radio Microwave Infrared Visible Ultraviolet X-ray γ-ray

  49. Spectroscopy and Ghosts Isaac Newton coined the term “spectrum”, based on the Latin “spectre”, meaning ghosts

  50. Origins of Spectroscopy • 1666 – Isaac Newton’s discovery that white light is composed of multiple colors • 1814 - Joseph Fraunhofer took a spectrum of the sun and discovered dark lines • 1857 - Gustav Kirchhoff and Robert Bunsen discovered that each element has a unique spectral signature