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Chapter 4: Light. “ Ever since we crawled out of that primordial slime, that’s been our unifying cry, ’More light.’ Sunlight. Torchlight. Candlelight. Neon, incandescent lights that banish the darkness from our caves to illuminate our roads, the insides - PowerPoint PPT Presentation
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Chapter 4: Light
“Ever since we crawled out of that primordialslime, that’s been our unifying cry, ’More light.’Sunlight. Torchlight. Candlelight. Neon, incandescent lights that banish the darkness from our caves to illuminate our roads, the insidesof our refrigerators...Little tiny flashlights forthose books we read under the covers whenwe’re suppose to be asleep. Light is more thanwatts ... Light is metaphor. Light is knowledge. Light is life...”
- Diane Frolov and Andrew SchneiderNorthern Exposure,1993
What can we learn by analyzing starlight?
• A star’s chemical composition
• A star’s temperature
• A star’s speed and direction of motion
How fast does light move?
• 186,000 miles per second!
or
• 3 x 105 kilometers per second!
Nothing travels faster than light!
Particle or Wave ?
1905, Einstein verified that light sometimes behaves as a wave and sometimes as particles.
This is called the wave-particle duality of light!
Particle or Wave ?
Wave: Electromagnetic wave
Particles: Photons (packets of energy)
Units: 1 nanometer= 10-9 meters!
Different wavelengths correspond to different colors!
Light as a Wave
Light as a Wave
if wavelength↓, then frequency ↑
if frequency ↑, then energy ↑
Some Important Relationships
if wavelength ↓, then energy ↑
Concept Question
Which color light has more energy blue or red?
Blue coats beat the red coats!
If you pass white light through a prism, it separates into its component colors.
ROY G. BIV !!
long wavelengths short wavelengths
Spectrum
A hot object or a hot, dense gas produces a continuous spectrum -- a complete rainbow of colors without any specific spectral lines.
A hot, less dense gas, when heated, produces an emission line spectrum - a series of bright spectral lines against a dark background.
A cooler gas in front of a hot dense gas produces an absorption line spectrum - a series of dark spectral lines among the colors of the rainbow.
Each chemical element produces its own unique set of spectral lines when it burns
Spectral Lines
By observing the location of the emission and or the absorption lines, one can identify the element!!!!
Twinkle, twinkle little starI don’t wonder what you are.For by spectroscopic kenI know that you are hydrogen!
- anonymous
The Spectral Type of Stars
• stars are classified by their spectral types:
O B A F G K M
• hottest to coolest
• most massive to least massive
• shortest lifetime to longest lifetime
Stars are classified by their spectra as O, B, A, F,G, K, and M spectral types
Oh Be A Fine Guy (or Girl), Kiss Me
Figure 3.5
Electromagnetic Spectrum
Not all radiation can penetrate Earth’s atmosphere.
What is Color?
• Color of objects due to how much light is reflected, absorbed, or emitted.
Kelvin Temperature Scale
Kelvin Temperature = Celsius Temperature + 273
Peak color (wavelength) shifts to shorter wavelengths as the temperature increases
increasing temperature
Which is HOTTER???
Peak color (wavelength) shifts to shorter wavelengths as the temperature increases
Humans emit infrared
light!
The Doppler Shift
Christian Doppler 1842:
• the observed frequency of an object is affected by its apparent motion
• Doppler shift occurs when the source of the waves is moving with respect to the observer
• occurs for all waves (e.g. water, sound, light)
Sound Waves
low pitchlow frequencylong wavelength
high pitchhigh frequencyshort wavelength
The Doppler Shift
The Doppler ShiftLight Waves
A B
constant wavelength detected by both A and B
A detects longer wavelength redshift
B detects shorter wavelength blueshift
Stationary Star
A B
Star Moving to the Right
The Doppler ShiftIn Astronomy
Spectrum of Stationary Source
Spectrum of Approaching Source
Spectrum of Receding Source
400nm 550nm 700nm
Detection of Extrasolar Planets: Stellar Wobble
• gravity of the planet causes the star to wobble back and forth
• 1990s, used Doppler effect to detect stellar wobbles
Concept QuestionIf a star is moving away from us, which statement best describes what is happeningto the star’s light?
A) the light is blueshifted; we perceive that the wavelength increasesB) the light is redshifted; we perceive that the wavelength increasesC) the light is blueshifted; we perceive that the wavelength decreasesD) the light is redshifted;we perceive that the wavelength decreases
What can we learn by analyzing starlight?
• A star’s chemical composition – by spectrum
• A star’s temperature – by color (peak wavelength)
• A star’s speed and direction of motion – by spectrum and Doppler Shift
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