© 2005 Pearson Education Inc., publishing as Addison-Wesley Light: The Messenger

© 2005 Pearson Education Inc., publishing as Addison-Wesley Homework #3 Due Friday, covers Chapt Problems - Some Quantitative with equations Telescope on Campbell: Tue & Thu 7-8 Midterm: Thursday, February 26 (14 days)

© 2005 Pearson Education Inc., publishing as Addison-Wesley What is a Planet Made of ? Chemical Composition of its surface and atmosphere ? Temperature = ? Is it: Ice ? Liquid? Gas? 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 the far distant Solar System. Discovered: April 2006 ( Mike Brown ). Artist’sRendering

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light in Everyday Life and in the Universe 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: Energy per Second Power: the rate at which energy is emitted or used. Measured in units called watts. 1 watt = 1 joule per second A 100 watt light bulb emits 100 joules of energy* every second. 1 kilowatt-hour: 1000 Joules/sec during 1 hour * Light Energy

© 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. 300,000 km/s Light travels at the speed of … Light: 300,000 km/s

© 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 Demonstration

© 2005 Pearson Education Inc., publishing as Addison-Wesley Light as Photons Light can also act as photons – particles of light energy. The energy carried by each photon depends on its frequency (color) E = hf = hc / [“h” is called Planck’s Constant] Bluer light carries more energy per photon.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Demo: Solar Cell - photons carry energy, absorbed by solar panel: makes a motor turn

© 2005 Pearson Education Inc., publishing as Addison-Wesley Domains of Wavelengths of Light “Electromagnetic spectrum” : Light waves of all wavelengths Gamma Rays (shortest wavelength: meter) X-Rays Ultraviolet Visible (0.5 x meters) Infrared (1-10 x10 -6 meters) Radio Waves (0.001 meter 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. Energy is carried by light. 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 Light and Matter: Atoms and Molecules emit light at specific wavelengths. Each atom and molecule emits a unique set of wavelengths. Solid Objects Emit “Thermal radiation”: Light at all wavelengths.

© 2005 Pearson Education Inc., publishing as Addison-Wesley How Atoms and Molecules Emit Light

© 2005 Pearson Education Inc., publishing as Addison-Wesley EmissionSpectra 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. A hot gas of one type of atom emits photons having wavelengths specific to that atom: Emission line spectrum.

© 2005 Pearson Education Inc., publishing as Addison-Wesley Demonstration: H and Ne Emission Tubes Use diffraction gratings on tubes, computer screen, laser pointer

© 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 When light shines through a gas, atoms will absorb those photons whose wavelengths match the atom’s electron energy levels. The resulting spectrum has all wavelengths (all colors), but is missing those 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 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 Warm, Solid Objects Emit Light: “Thermal Emission” Examples: Electric Stove Filaments Hot fireplace Coals Light bulb filament

© 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 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).