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

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

Light: The Messenger

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• Homework #3 Due Friday, covers Chapt. 5-8

• 6 Problems - Some Quantitative with equations

• Telescope on Campbell: Tue & Thu 7-8

• Midterm: Thursday, February 26 (14 days)




What is a Planet Made of ?What is a Planet Made of ?

• Chemical Composition of its surface and atmosphere ?

• Temperature = ?• Is it:

Ice ?

Liquid?

Gas?



The “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’sArtist’sRenderingRendering


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


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

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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 … Light: 300,000 km/s300,000 km/s


Light as a Wave

A wave is a pattern which is revealed by its interaction with particles.


Properties of a Wave


Light as a WaveLight 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.

• Our eyes recognize f (or ) as ccoolloorr .


PrismDemonstration


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.


Demo:

Solar Cell -photons carry energy,

absorbed by solar panel:makes a motor turn


Domains of Wavelengths of Light

• “Electromagnetic spectrum” :

Light waves of all wavelengths

Gamma Rays (shortest wavelength: 10-10 meter)

X-Rays

Ultraviolet

Visible (0.5 x 10-6 meters)

Infrared (1-10 x10-6 meters)

Radio Waves (0.001 meter or more)


The Electromagnetic SpectrumMost wavelengths of light can not be seen by the human eye.




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


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.


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.


How Atoms and MoleculesEmit Light


EmissionSpectrafrom 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.


Demonstration: H and Ne

Emission TubesUse diffraction gratings

on tubes, computer screen, laser pointer


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.

HydrogenAtom


Absorption of Lightby atoms or molecules


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 You can determine which atoms are in an object by the emission & absorption lines in the spectrum.emission & absorption lines in the spectrum.


Warm, Solid Objects Glow by

Thermal Emission of Light



Cool Warmer Hot HotterCool Warmer Hot HotterRed & Faint White & BrightRed & Faint White & Bright


Warm, Solid Objects Emit Light:“Thermal Emission”

Examples:Examples:

Electric Stove Electric Stove FilamentsFilaments

Hot fireplace Hot fireplace CoalsCoals

Light bulbLight bulbfilamentfilament


Light carries informationabout the Planets and Stars

By studying the spectrum of an object, we can learn its:1 Composition2 Temperature3 Velocity

Key: Separate light into its different wavelengths (spectrum).


Summary of Demos:• E+50+20 Prism - to be used with overhead projector• E + 15 + 25 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 + 15 + 25• 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+15+20 Solar-cell-powered helicopter• Doppler: spin buzzer overhead.

More gratings are in 264 Evans (TALC room).

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

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