Light and Matter

1. Light and Matter

2. Light in Everyday Life Our goals for learning: • How do we experience light? • How do light and matter interact? The warmth of sunlight tells us that light is a form of energy. Energy is measured in joules. We can measure the flow of energy in light in units of watts: 1 watt = 1 joule/s

3. Interactions of Light • 4 process: • Emission • Absorption • Transmission • Transparent objects transmit (allow to pass) light • Opaque objects block (absorb) light • Reflection or Scattering • White light is made up of many different colors

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

5. Interactions of Light with Matter Interactions between light and matter determine the appearance of everything around us. Objects with color (e.g. a red rose) appear that color because they absorb all the other colors and reflect (or scatter) that one.

6. What is light? • Light is a form of energy that can act either like a wave or like a particle (with energy and momentum) depending on its interaction with matter • A light wave is a vibration of electric and magnetic fields – an electromagnetic wave • Particles of light are bundles (quanta) of energy called photons

7. Waves • A wave is a pattern of motion that can carry energy without carrying matter along with it Leave bobs up and down

8. 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 (cycles per second or hertz) wave speed = wave length x frequency Eyes are sensitive to wavelength; we sense wavelength differences as “color”. Mathematically: c = f : the Greek letter lambda

9. Particles of Light • Particles of light are called photons • Each photon has a wavelength and a frequency associated with it • The Energy of a photon depends on its frequency, E = hf • h is a constant of nature called Planck’s constant

10. Wavelength, Frequency, and Energy l xf = c • = wavelength, f = frequency Visible Light:= 0.5 x 10-6 m, f = 6 x 1014 s-1 (Hz) c = 3.00 x 108 m/s = speed of light E = h xf = photon energy h = 6.626 x 10-34 joule x s = Planck’s constant One photon of visible light carries ~4 x 10-19 joules of energy A 100W bulb emits 2.5 x 1020 photons every second!

12. What have we learned? • What is light? • Light can behave like either a wave or a particle • A light wave is a vibration of electric and magnetic fields • Light waves have a wavelength and a frequency • Photons are particles of light. • What is the electromagnetic spectrum? • Human eyes cannot see most forms of light. • The entire range of wavelengths of light is known as the electromagnetic spectrum.

13. 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? We need to know this in order to understand the end phase of a star’s life & what’s happening inside a white dwarf or neutron star.

14. What is the structure of matter? Everything is made of atoms Electrons have negative charge and are almost 2000x less massive than protons Electron Cloud 10-15 m Nucleus Atom Neutron:no charge Proton: positive charge Volume of atom = 1,000 trillion times that of nucleus

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

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

17. What are the phases of matter? • Phases: • Solid (ice) • Liquid (water) • Gas (water vapor) • Plasma (ionized gas) • Phases of same material behave differently because of differences in chemical bonds. By chemical bonds we mean the electric forces between atoms.

18. Phase Changes Read from bottom to top • Ionization: Stripping of electrons, changing atoms into plasma • Dissociation: Breaking of molecules into atoms • Evaporation: Breaking of flexible chemical bonds, changing liquid into gas • Melting: Breaking of rigid chemical bonds, changing solid into liquid

19. What have we learned? • What is the structure of matter? • Matter is made of atoms, which consist of a nucleus of protons and neutrons surrounded by a cloud of electrons • What are the phases of matter? • Adding heat to a substance changes its phase by breaking chemical bonds. • As temperature rises, a substance transforms from a solid to a liquid to a gas, then the molecules can dissociate into atoms • Stripping of electrons from atoms (ionization) turns the substance into a plasma

20. 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?

21. What are the three basic types of spectra? Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum Spectra of astrophysical objects are usually combinations of these three basic types. We can take a picture of a spectrum (lower bar) or we can plot a graph of intensity versus wavelength (upper).

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

23. Emission Line Spectrum Each colored “line” is an image of the entrance slit. • 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

24. 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

25. Chemical Fingerprints • Electrons in atoms can only occupy certain energy states or levels • The lowest energy state (level 1) is the Ground State • Downward transitions between energy states produce a unique pattern of emission lines (E = hf = hc/)

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

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

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

29. Energy Levels of Molecules • Molecules have additional energy levels because they can vibrate and rotate • The “spring” just represents the electrical bond between the atoms of the molecule

30. Energy Levels of Molecules • The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated • Many of the energy transitions due to vibration and rotation of molecules occur in the infrared part of the spectrum

31. Thermal Radiation • Nearly all large or dense objects emit thermal radiation, including stars, planets, you… • Collisions between atoms in a hot object causes electrons to jump to higher energy levels for a while and then drop down again to emit light. As a result, the photons produced are intimately linked with the temperature (average kinetic energy) in the collisions. Radiation produced this way is called thermal. • An object’s thermal radiation spectrum depends on only one property: its temperature

32. Properties of Thermal Radiation Hotter objects emit more light at all frequencies per unit area. Power per sq. meter = σT4 (Stefan’s Law) Hotter objects emit photons with a higher average energy. maxT ~ 3000 (for  in m) (Wien’s Law) Larger objects can emit more total light even if they are cooler. For a sphere (star), luminosity is L = 4πR2σT4

33. Thought QuestionWhy don’t we glow in the dark? • People do not emit any kind of light. • People only emit light that is invisible to our eyes. • People are too small to emit enough light for us to see. • People do not contain enough radioactive material.

34. Thought QuestionWhy don’t we glow in the dark? • People do not emit any kind of light. • People only emit light that is invisible to our eyes. We glow in the infrared. • People are too small to emit enough light for us to see. • People do not contain enough radioactive material.

35. What have we learned? • What are the three basic type of spectra? • Continuous spectrum, emission line spectrum, absorption line spectrum • How does light tell us what things are made of? • Each atom has a unique fingerprint. • We can determine which atoms something is made of by looking for their fingerprints in the spectrum.

36. What have we learned? • How does light tell us the temperatures of planets and stars? • Nearly all large or dense objects emit a continuous spectrum that depends on temperature. • The spectrum of that thermal radiation tells us the object’s temperature. • How do we interpret an actual spectrum? • By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

37. The Doppler Effect Our goals for learning: • How does light tell us the speed of a distant object? • How does light tell us the rotation rate of an object?

38. How does light tell us the speed of a distant object? The Doppler Effect Waves are compressed in the direction of motion  wavelength is decreased frequency is higher Same thing happens for light.

39. Measuring the Shift Stationary • We generally measure the Doppler Effect from shifts in the wavelengths of spectral lines • The fractional shift is: ( - 0)/0 where 0 is the undisturbed wavelength; this number is equal to the speed of the object relative to that of light (V/c) Moving Away Away Faster Moving Toward Toward Faster

40. Doppler shift tells us ONLY about the part of an object’s motion toward or away from us: Pure radial motion – maximum Doppler shift Transverse motion – no Doppler shift Part radial, part transverse – Doppler shift gives Vr = Vcosθ (less than V) Vr θ

41. Thought QuestionI measure a spectral line in the lab at 500.7 nm.The same line in a star has wavelength 502.8 nm. What can I say about this star? • It is moving away from me. • It is moving toward me. • It has unusually long spectral lines.

42. Thought QuestionI measure a spectral line in the lab at 500.7 nm.The same line in a star has wavelength 502.8 nm. What can I say about this star? • It is moving away from me. This is a REDSHIFT • It is moving toward me. • It has unusually long spectral lines. And redshift = (502.8 – 500.7)/500.7 = 0.004194 Therefore the radial component of velocity = 0.004194 x c = 1,258 km/s

43. Measuring Redshift

44. How does light tell us the rotation rate of an object? Spectrum of a Rotating Object Slower Faster Spectral lines are wider when an object rotates faster

45. What have we learned? • How does light tell us the speed of a distant object? • The Doppler effect tells us how fast an object is moving toward or away from us. • Blueshift:objects moving toward us • Redshift: objects moving away from us • How does light tell us the rotation rate of an object? • The width of an object’s spectral lines can tell us how fast it is rotating

46. Telescopes: Portals of Discovery

47. Chile 4 x 8m Mauna Kea, Hawaii A Selection of Major Telescopes 2 x 10m X-ray telescope in space Hubble: 2.5m in space New Mexico VLA:27 radio telescopes

48. Eyes and Cameras: Everyday Light Sensors Our goals for learning: • How does your eye form an image? • How do we record images?

49. How does your eye form an image? The lens bends (refracts) light rays and brings to a focus on retina.

50. Refraction • Refraction is the bending of light when it passes from one substance into another • Your eye uses refraction to focus light