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
of our refrigerators...Little tiny flashlights for
those books we read under the covers when
we’re suppose to be asleep. Light is more than
watts ... Light is metaphor. Light is knowledge.
Light is life...”
- Diane Frolov and Andrew Schneider
Northern 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 waveparticle duality of light!
Particle or Wave ?
Wave:
Electromagnetic
wave
Particles:
Photons (packets of
energy)
Light as a Wave
Different wavelengths
correspond to different colors!
Units: 1 nanometer= 10-9 meters!
Light as a Wave
Some Important Relationships
if wavelength↓, then frequency ↑
if frequency ↑, then energy ↑
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.
Spectrum
ROY G. BIV !!
long wavelengths
short wavelengths
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 star
I don’t wonder what you are.
For by spectroscopic ken
I know that you are hydrogen!
- anonymous
The Spectral Type of Stars
• stars are classified by their spectral types:
O BAFGK 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)
The Doppler Shift
Sound Waves
low pitch
low frequency
long wavelength
high pitch
high frequency
short wavelength
The Doppler Shift
Light Waves
Stationary Star
A
Star Moving to the Right
B
constant wavelength detected
by both A and B
A
B
A detects longer wavelength  redshift
B detects shorter wavelength blueshift
The Doppler Shift
In Astronomy
Spectrum of Approaching Source
Spectrum of Stationary 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 Question
If a star is moving away from us, which
statement best describes what is happening
to the star’s light?
A) the light is blueshifted;
we perceive that the wavelength increases
B) the light is redshifted;
we perceive that the wavelength increases
C) the light is blueshifted;
we perceive that the wavelength decreases
D) 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