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

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

Lecture 8

Light: The Cosmic Messenger

Geoff Marcy

• Read Chapter 5 : “Light”

• Homework: MasteringAstronomy

Assignment Chapter 5

• Midterm: Next Tuesday, Sept. 30 (in 1 week). - Covers Chapters 1-5 - 30 Multiple Choice Questions (We provide scantron forms.) No calculators needed.

- Questions are based on: - Book - Lectures - Homework

Observing Project #1: Monitoring the Motion of MarsChart the position of Mars in the night sky relative to Saturn and the star, Antares.Observe Mars, Saturn, and Antares in the evening sky three times (once per week), between Sept 9 and October 1.

Write one to two paragraphs explaining your observations of the position of Mars. Did it move? If so, why did it move the way it did?

Due Thursday, October 2.

Sept. 5, 2014

Look just to the left of San Francisco

.

..Where is Mars?


• Light carries energy per second: Watts• It has wavelengths, frequencies• It exhibits the Doppler Effect

What is Light? A wave of electric and magnetic fields


How do you learn

What a Planet is Made of ?

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 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?


Light in Everyday Life and in the Universe

• A Type of Energy that Moves through space• Composed of different wavelengths (colors)

Where does light come from?• Atoms and Molecules Emit Light• Atoms and Molecules Absorb Light


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• 1 kilowatt-hour = 3,600,000 Joules

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/sLight


Light as a Wave

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


Properties of a Wave


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.

• Our eyes recognize f (or ) as color .


PrismA Spectrometer

The Doppler Effect

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

The Doppler Effect


The Doppler Effect


The Doppler Effect

velocity c

=

Change in Wavelength

Wavelength


Section 2Lecture 7


• Light as “Photons”:• Atoms Emit and Absorb Light


Light as Photons

• The energy carried by each photon depends

on its frequency and wavelength (color):

Energy of a Photon = hf = hc / [“h” is called Planck’s Constant]

• Bluer light carries more energy per photon.• Redder light carries less energy per photon.


Photons carry energy.

When absorbed by a solar panelphotons generate electricity




Domains of Wavelengths of Light

• “Electromagnetic spectrum” : Light waves of all wavelengths:

Type WavelengthGamma Rays (shortest wavelength: 10-10 meter)X-Rays 10-10 - 10-8 metersUltraviolet 0.01 - 0.5 x 10-6 metersVisible 0.4 - 0.7 x 10-6 metersInfrared 1-100 x10-6 metersRadio Waves 1 millimeter or more


The Electromagnetic Spectrum

Most wavelengths of light can not be seen by the human eye.


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


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.


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.


Emission Spectrafrom 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.


How Atoms and MoleculesEmit Light


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

• 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.



• Thermal Emission of Light

by Warm Bodies


Warm, Solid Objects Glow by

Thermal Emission of Light

Cool Warmer Hot HotterRed & Faint White & Bright


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 Temp4 )

3. Hotter objects emit higher energy photons (bluer) average increases as 1/ T (using kelvin Temp scale)

“Thermal Emission” from Warm, Opaque Objects


Dog

Thermal EmissionAt Infrared Wavelengths


Warm, Solid Objects Emit Light:“Thermal Emission”

Examples:

• Electric Stove Filaments

• Fireplace Coals

• Light bulb filament

• Warm human body


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).


Spectrum from a Typical Planet, Comet, or Asteroid

Spectrum reveals:1 Chemical Composition2 Temperature3 Velocity (from the Doppler effect)

Reflected visible light from Sun Thermal Emission (IR)

Absorption by moleculesin gases in atmosphere


End of Lecture 8


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).

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