© 2005 Pearson Education Inc., publishing as Addison-Wesley Lecture 8 Light: The Cosmic Messenger Geoff Marcy

© 2005 Pearson Education Inc., publishing as Addison-Wesley Read Chapter 5 : “Light” Homework: MasteringAstronomy Assignment Chapter 5 Midterm: - Tuesday, October 1 (in 1 week). - Covers Chapters Multiple Choice & 2 Short-answer problems

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light carries energy: Watts Wavelengths, frequencies The Doppler Effect What is Light? A wave of electric and magnetic fields

© 2005 Pearson Education Inc., publishing as Addison-Wesley How do you learn What a Planet is Made of ? T he “Dwarf Planet”, Eris, and its moon, Dysnomia. It is twice as far as Pluto and is larger than Pluto. Numerous other icy objects larger than Pluto likely exist in the Kuiper Belt of comets of the far distant Solar System. Discovered: April 2006 (Mike Brown). Artist’sRendering Chemical Composition of its surface and atmosphere ? Temperature = ? Is it: Rock or Ice ? Liquid? Gas?

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light in Everyday Life and in the Universe A Type of Energy that Moves through space Composed of different wavelengths (colors) Atoms and Molecules Emit Light Atoms and Molecules Absorb Light

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light Power: Amount of energy emitted per second Power: the rate at which energy is emitted or used. Measured in units: watts. 1 watt = 1 joule of energy per second A 100 watt light bulb emits 100 joules of energy every second. 1 kilowatt-hour = ??? Joules? Hint: 3600 seconds in 1 hour 3,600,000 Joules

© 2005 Pearson Education Inc., publishing as Addison-Wesley Properties of Light Light can act as a wave: “Electromagnetic wave” Light can act as a particle: “photon” Light has wavelength and frequency. Light travels at the speed of … c = 300,000 km/s c = 300,000 km/s Light

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light as a Wave A wave is a pattern which is revealed by its interaction with particles.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Properties of a Wave

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light as a Wave f = frequency  = wavelength wave speed = f Speed of light is “c” = 300,000 km/s For light: f = c The higher f is, the smaller is, and vice versa. colorOur eyes recognize f (or ) as color.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Prism A Spectrometer

The Doppler Effect Waves emitted from an object moving towards you will have its wavelength shortened. The Doppler Effect

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Doppler Effect

© 2005 Pearson Education Inc., publishing as Addison-Wesley

The Doppler Effect  velocity c = Change in Wavelength Wavelength

© 2005 Pearson Education Inc., publishing as Addison-Wesley Section 2 Lecture 7 Light: The Cosmic Messenger Light as “Photons”: Atoms Emit and Absorb Light

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light as Photons The energy carried by each photon depends on its frequency and wavelength (color) : Energy = hf = hc / [“h” is called Planck’s Constant] Bluer light carries more energy per photon. Redder light carries less energy per photon.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Photons carry energy. When absorbed by a solar panel photons generate electricity

© 2005 Pearson Education Inc., publishing as Addison-Wesley

Domains of Wavelengths of Light “Electromagnetic spectrum” : Light waves of all wavelengths: Type Wavelength Gamma Rays (shortest wavelength: meter) X-Rays meters Ultraviolet x meters Visible x meters Infrared x10 -6 meters Radio Waves 1 millimeter or more

© 2005 Pearson Education Inc., publishing as Addison-Wesley The Electromagnetic Spectrum Most wavelengths of light can not be seen by the human eye.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Summary Light A vibration in an electromagnetic field moving through space. Light has energy. Light as a wave Light as a particle E = hf photon f = c

© 2005 Pearson Education Inc., publishing as Addison-Wesley Atoms and Molecules Interact with Light 1. Emission – Atoms and molecules release energy as light 2. Absorption – Atoms and molecules absorb light energy 3. Reflection – Light bounces off matter.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Atoms and Molecules Emit Light Atoms and Molecules emit light at specific wavelengths. Each atom and molecule emits light at a unique set of wavelengths. Solid Objects Emit Light by “Thermal radiation”: Light Emitted at all wavelengths.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Emission Spectra from atoms and molecules Each type of atoms has a unique set of electron energy levels. Each atoms emits its own set of wavelengths, Like fingerprints. Emission line spectrum.

© 2005 Pearson Education Inc., publishing as Addison-Wesley

How Atoms and Molecules Emit Light

© 2005 Pearson Education Inc., publishing as Addison-Wesley Energy Levels of Atoms Electron is allowed to have certain energies in an atom. Electrons can absorb light and gain energy or emit light when they lose energy. Consider light as a photon when discussing its interaction with matter. Only photons whose energies (colors) equal the difference in electron energy levels can be emitted or absorbed. Hydrogen Atom

© 2005 Pearson Education Inc., publishing as Addison-Wesley Absorption of Light by atoms or molecules

© 2005 Pearson Education Inc., publishing as Addison-Wesley Absorption of Light by Atoms & Molecules Atoms absorb photons whose energies (i.e. wavelengths) match the energy difference between two levels in an atom. The resulting spectrum has all wavelengths (all colors), but is missing wavelengths that were absorbed. You can determine which atoms are in an object by the emission & absorption lines in the spectrum.You can determine which atoms are in an object by the emission & absorption lines in the spectrum.

© 2005 Pearson Education Inc., publishing as Addison-Wesley End of section 2 End of section 2

© 2005 Pearson Education Inc., publishing as Addison-Wesley Section 3 Lecture 7 Light: The Cosmic Messenger Thermal Emission of Light by Warm Bodies

© 2005 Pearson Education Inc., publishing as Addison-Wesley Warm, Solid Objects Glow by Thermal Emission of Light Cool Warmer Hot Hotter Cool Warmer Hot Hotter Red & Faint White & Bright

© 2005 Pearson Education Inc., publishing as Addison-Wesley 1. Warm objects emit Infrared light and radio waves Examples: Warm embers of fire, electric stove. 2. Hotter objects emit more light energy per unit surface area (per second). (Energy increases as Temp 4 ) 3. Hotter objects emit higher energy photons (bluer) average increases as 1/ T (using kelvin Temp scale) “Thermal Emission” from Warm, Opaque Objects

© 2005 Pearson Education Inc., publishing as Addison-Wesley Dog Thermal Emission At Infrared Wavelengths

© 2005 Pearson Education Inc., publishing as Addison-Wesley Warm, Solid Objects Emit Light: “Thermal Emission” Examples: Electric StoveElectric Stove Filaments Filaments Fireplace CoalsFireplace Coals Light bulb filamentLight bulb filament Warm human bodyWarm human body

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light carries information about the Planets and Stars By studying the spectrum of an object, we can learn its: 1Composition 2Temperature 3Velocity Key: Separate light into its different wavelengths (spectrum).

© 2005 Pearson Education Inc., publishing as Addison-Wesley Spectrum from a Typical Planet, Comet, or Asteroid Spectrum reveals: Spectrum reveals: 1Chemical Composition 2Temperature 3Velocity (from the Doppler effect) Reflected visible light from Sun Thermal Emission (IR) Absorption by molecules in gases in atmosphere

© 2005 Pearson Education Inc., publishing as Addison-Wesley End of Lecture 7

© 2005 Pearson Education Inc., publishing as Addison-Wesley Summary of Demos: E Prism - to be used with overhead projector E Discharge tubes: view with diffraction gratings, Neon and hydrogen. Can tell composition from pattern of lines. include: 100 slide-mounted diffraction gratings & 50 stick-mounted gratings, all in the physics stockroom under E Comes with E+50_20: Carbon or Tungsten lamp with variac: change temp. and view with the diffraction gratings to see blue emerge. Blackbody radiation: hot --> blue F Solar-cell-powered helicopter Doppler: spin buzzer overhead. More gratings are in 264 Evans (TALC room).