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Chapter 5 Light and Matter: Reading Messages from the Cosmos

Chapter 5 Light and Matter: Reading Messages from the Cosmos. 5.1 Light in Everyday Life. Our goals for learning: How do we experience light? How do light and matter interact?. How do we experience light?. The warmth of sunlight tells us that light is a form of energy.

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Chapter 5 Light and Matter: Reading Messages from the Cosmos

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  1. Chapter 5Light and Matter: Reading Messages from the Cosmos

  2. 5.1 Light in Everyday Life Our goals for learning: • How do we experience light? • How do light and matter interact?

  3. How do we experience light? • The warmth of sunlight tells us that light is a form of energy. • We can measure the flow of energy in light in units of watts: 1 watt = 1 joule/s.

  4. Colors of Light • White light is made up of many different colors.

  5. How do light and matter interact? • Emission • Absorption • Transmission • Transparent objects transmit light. • Opaque objects block (absorb) light. • Reflection/scattering

  6. Reflection and Scattering Mirror reflects light in a particular direction. Movie screen scatters light in all directions.

  7. Interactions of Light with Matter Interactions between light and matter determine the appearance of everything around us.

  8. Thought QuestionWhy is a rose red? • The rose absorbs red light. • The rose transmits red light. • The rose emits red light. • The rose reflects red light.

  9. Thought QuestionWhy is a rose red? • The rose absorbs red light. • The rose transmits red light. • The rose emits red light. • The rose reflects red light.

  10. 5.2 Properties of Light Our goals for learning: • What is light? • What is the electromagnetic spectrum?

  11. What is light? • Light can act either like a wave or like a particle. • Particles of light are called photons.

  12. Waves • A wave is a pattern of motion that can carry energy without carrying matter along with it.

  13. Properties of Waves • Wavelength is the distance between two wave peaks. • Frequency is the number of times per second that a wave vibrates up and down. Wave speed = wavelength  frequency

  14. Light: Electromagnetic Waves • A light wave is a vibration of electric and magnetic fields. • Light interacts with charged particles through these electric and magnetic fields.

  15. Wavelength and Frequency wavelength  frequency = speed of light = constant

  16. Particles of Light • Particles of light are called photons. • Each photon has a wavelength and a frequency. • The energy of a photon depends on its frequency.

  17. Wavelength, Frequency, and Energy l f = c l = wavelength, f = frequency c = 3.00  108 m/s = speed of light E = h f = photon energy h = 6.626  10-34 joule  s = photon energy

  18. Special Topic: Polarized Sunglasses • Polarization describes the direction in which a light wave is vibrating. • Reflection can change the polarization of light. • Polarized sunglasses block light that reflects off of horizontal surfaces.

  19. What is the electromagnetic spectrum?

  20. Types of light (from lowest to highest energy): Radio waves: wavelengths kms to mm (microwaves) Infrared (IR): wavelengths mm to 10-6 meters (microns) Visible light (the only light our eyes can see) 700 nm (nanometers) = 700 x 10-9 meters = 7 x 10-7 m reddest light most of us can see to 400 nm = bluest light most of us can see

  21. Ultra-violet light: 10-8 meters, or about the size of atoms. Energies are now high enough to cause sunburn, melanomas X-rays: 10-11 meters; energies high enough to pass through most tissue Gamma-rays: 10-16 meters (energetic enough to cause cell damage with relatively low exposure)

  22. The Electromagnetic spectrum

  23. Objects can look very different depending on the wavelength of light you are detecting: Sun as seen in visible, UV, X-ray and radio light

  24. Thought QuestionThe higher the photon energy, • the longer its wavelength. • the shorter its wavelength. • energy is independent of wavelength.

  25. Thought QuestionThe higher the photon energy, • the longer its wavelength. • the shorter its wavelength. • energy is independent of wavelength.

  26. 5.3 Properties of Matter Our goals for learning: • What is the structure of matter? • What are the phases of matter • How is energy stored in atoms?

  27. What is the structure of matter?

  28. Atomic Terminology • Atomic number = # of protons in nucleus • Atomic mass number = # of protons + neutrons • Molecules: consist of two or more atoms (H2O, CO2)

  29. Atomic Terminology • Isotope: same # of protons but different # of neutrons (4He, 3He)

  30. What are the phases of matter? • Familiar phases: • Solid (ice) • Liquid (water) • Gas (water vapor) • Phases of same material behave differently because of differences in chemical bonds.

  31. Phase Changes • Ionization: stripping of electrons, changing atoms into plasma • Dissociation: breaking of molecules into atoms • Evaporation: breaking of flexible chemical bonds, changing liquid into solid • Melting: breaking of rigid chemical bonds, changing solid into liquid

  32. Phases and Pressure • Phase of a substance depends on both temperature and pressure. • Often more than one phase is present.

  33. How is energy stored in atoms? • Electrons in atoms are restricted to particular energy levels. Excited states Ground state

  34. Energy Level Transitions • The only allowed changes in energy are those corresponding to a transition between energy levels.

  35. 5.4 Learning from Light Our goals for learning: • What are the three basic types of spectra? • How does light tell us what things are made of? • How does light tell us the temperatures of planets and stars? • How do we interpret an actual spectrum?

  36. What are the three basic types of spectra? Spectra of astrophysical objects are usually combinations of these three basic types.

  37. Three Types of Spectra

  38. Continuous Spectrum • The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.

  39. Emission Line Spectrum • A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines.

  40. Absorption Line Spectrum • A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum.

  41. How does light tell us what things are made of?

  42. Chemical Fingerprints • Each type of atom has a unique set of energy levels. • Each transition corresponds to a unique photon energy, frequency, and wavelength. Energy levels of hydrogen

  43. Chemical Fingerprints • Downward transitions produce a unique pattern of emission lines.

  44. Chemical Fingerprints • Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.

  45. Chemical Fingerprints • Each type of atom has a unique spectral fingerprint.

  46. Chemical Fingerprints • Observing the fingerprints in a spectrum tells us which kinds of atoms are present.

  47. Energy Levels of Molecules • Molecules have additional energy levels because they can vibrate and rotate.

  48. Energy Levels of Molecules • The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated. • Many of these molecular transitions are in the infrared part of the spectrum.

  49. Thought QuestionWhich letter(s) label(s) absorption lines?

  50. Thought QuestionWhich letter(s) label(s) absorption lines?

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