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What is Light?
● Newton– Prism shows white light contains all colors– Light made of particles (photons)
● Maxwell– Theory of electricity and magnetism– Light is electromagnetic waves
● Produced by wiggling electrons● Radiation = production of light
● Quantum Mechanics– Light is both: particle and wave
Waves● Wavelength ( )
– Distance between crests (or troughs)● Frequency ( f )
– How often it repeats (wiggles up and down) ● Measured in Hertz (Hz)
– number of times per sec
Waves● Speed c = 3 x 108 m/s
c = f● Wavelength inversely related to frequency
= c / f– high frequency = short wavelength
– low frequency = long wavelength
Properties of Light
● Color– Depends on frequency
● blue = high frequency = short wavelength● red = low frequency = long wavelength
● Carries energy (heat)– Photon energy
E = h f● high frequency = high energy = blue● low frequency = low energy = red
h = Planck’s constant
Red light has ____ than blue light.
A. larger frequency, energy, and wavelengthB. smaller frequency, energy, and wavelengthC. larger frequency and energy, but smaller wavelengthD. smaller frequency and energy, but larger wavelength
Which of the following travels fastest?
A. radio waves B. infrared (heat) wavesC. microwavesD. blue light wavesE. none of the above
} All are types oflight!
All types of light travel at the same speed - the “speed of light”, c
CPS Question
● The color of visible light is determined by its ____.
● A)brightness● B) amplitude● C)speed● D)wavelength
CPS Question
● If the wavelength of light increases, the frequency must ____.
● A)increase also● B) decrease● C)remain unchanged
CPS Question
● The bending of light that occurs when moving between media of different densities is called ___.
● A)reflection● B) refraction● C)diffraction● D)distortion
Propagation of Light● Photons travel in straight lines
– energy spread over larger area at larger distances
– produces 1/r2 decrease in brightness● Double distance - brightness decreases by 4
If a 100-watt light bulb is placed 10 feet away fromyou, and an identical 100-watt light bulb is placed 100 feet away from you, which will appear brighter?
A. The closer oneB. The farther oneC. They will appear the same brightness
How much fainter will the far one appear compared to the close one?
A. Twice as faintB. 10 times fainterC. 100 times fainterD. 1000 times fainter
~ 1/r2
Electromagnetic Spectrum
● Visible light:– red, orange, yellow, green, blue,
indigo, violet (ROYGBIV)● Invisible Light:
– Ultraviolet = bluer than blue
– Infrared = redder than red
– Other wavelengths:● Short: X-rays, gamma-rays● Long: microwave, radio
Thermal Radiation● All objects radiate (thermal radiation)
– Objects made of atoms– Atoms (and their electrons) vibrate
● Wiggling electrons radiate, producing light– Bigger objects produce more light– Higher temperature = stronger vibration
● Hotter objects emit more light● Perfect absorber is black
– Absorbed light (energy) heats object– Temperature increases until
emitted energy = absorbed energy– Emitted radiation called Blackbody Radiation
● Thermal radiation emitted by most objects similar to blackbody
What does the spectrum of an astronomical object's radiation look like?
Many objects (e.g. stars) have roughly a "Black-body" spectrum:
Brightness
Frequency
also known as the Planck spectrum or Planck curve.
Blackbody Radiation Laws
● Luminosity, L
L = energy emitted per second● Luminosity for a spherical object (a star)
L = 4R2 T4
R = radius (size) of star; T = temperature
– double size, luminosity increases by 2x2 = 4
– double temperature; luminosity increases by 2x2x2x2 = 16
Stefan-Boltzmann Law
Questions
● How is temperature related to the amount of energy radiated?
● How is temperature related to the color of the object?
(Blackbody Demo)
"cold" dust
"hot" stars
"cool" star
Sun
frequency increases, wavelength decreases
The wavelength of peak emission tells us the temperature of the object!
Blackbody Radiation Laws
● Color– Wavelength where most light emitted
max = 3 x 106 / T
T in Kelvin; max in nanometers (1 nm=10-9m)
● Cool stars are red● Hot stars are blue
– Color indicates temperature!
Wien’s Law
As T , Wavelength , Color = redder As T , Wavelength , Color = bluer
The graph above shows blackbody spectra for three different stars. Which of the stars is at the highest temperature?
A. Star AB. Star BC. Star C
Because peak energy emissionoccurs at shortest wavelength
Doppler Shift
● Originally discovered using sound waves● Moving object
– emits light with slightly different color● Frequency (pitch) of approaching object is higher
– Blueshift● Wavelength shorter (shifted blueward)
● Frequency (pitch) of receeding object is lower– Redshift
● Wavelength longer (shifted redward) video
Video
Spectroscopy
● Prism separates light into different colors– Continuous spectrum
● contains all colors● Example: blackbody spectrum
Spectroscopy
Absorption Line Spectrum– Some colors are
missing (discrete lines)
N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
SolarSpectrum
Spectroscopy– Emission Line spectrum
● Only certain colors are present (discrete lines)● Spectrum for each element unique (like fingerprints)
For a given element, emission and absorption lines occur at the same wavelengths.
Helium discovered in Sun’s spectrum before being found on Earth!
Sodium emission and absorption spectra
Spectrum of the Sun• Absorption spectrum
• What causes emission/absorption of light at specific wavelengths?
Interactive
Video 1, 2, 3
The Particle Nature of Light
Light interacts with matter as individual packets of energy, called photons.
c
photon energy is proportional to frequency:
E f (or E 1
example: ultraviolet photons are more harmful than visible photons.
Model Atom● Nucleus
– contains protons and neutrons– number of protons = element
(1 proton = hydrogen, 2 protons = helium, etc.)– number of neutrons about same as protons
● Isotope = different number of neutrons
hydrogen
helium
Isotopes of hydrogen
Model Atom● Electrons orbit nucleus
– Number of electrons = number of protons● Ionization = removing electrons
– Only certain orbits are allowed
hydrogen helium
The Nature of Atoms
The Bohr model of the Hydrogen atom:
_
+
proton
electron
"ground state"
_
+
an "excited state"
(Fair Analogy)
Atomic Absorption● Atom absorbs photon energy
– electron “jumps” to higher energy orbit
– only certain discrete orbits are allowed● Atom can absorb only discrete colors (energies)
When an atom absorbs a photon, it moves to a higher energy state briefly
When it jumps back to lower energy state, it emits photon(s) in a random direction, conserving the total energy of the system!
Atomic Emission● Electron “jumps” to a lower energy orbit
– Atom emits photon
– can emit only discrete colors ● same colors (wavelengths/energies) as absorption
Atomic Energy Levels
● Energy Levels– Different for each element
● each element has unique set of absorption/emission lines
Other elements
Helium Carbon
neutron proton
Each element has its own allowed energy levels yielding a unique spectral fingerprint.
Kirchoff’s Laws
● Continuous spectrum– Produced by hot solid (or dense gas)
● Emission line spectrum– Produced by hot, low density gas
● Absorption line spectrum– Produced when continuous source is viewed through
cooler low density gas
Kirchoff’s Laws
● Absorption lines same wavelengths as emission lines– Gas can only absorb and emit at certain discrete
frequencies/wavelengths/energiesvideo
If you analyze the light from a low density object (such as a cloud of interstellar gas), which type of spectrum do you see?
A. dark line absorption spectrumB. bright line emission spectrumC. continuous spectrum
Imagine that you observe the Sun while in your space ship far above Earth’s atmosphere. Which of the following spectra would you observe by analyzing the sunlight?
A. dark line absorption spectrumB. bright line emission spectrumC. continuous spectrum
CPS Question Which ONE of these is constant for all forms of
EM radiation in a vacuum?
A) amplitude B) wavelength C) frequency D) speed E) energy
CPS Question
Which ONE is NOT a property of a blackbody?
A) It appears black, regardless of its temperature. B) It emits radiation in a continuum of
wavelengths. C) Its spectrum peaks at a wavelength determined
by its temperature. D) The total energy that it radiates increases rapidly
with temperature.
CPS Question
The Sun's observed spectrum is _____.
A) A continuum with no lines, like the rainbow. B) A continuum with bright emission lines. C) Only absorption lines on a black background. D) Nearly a continuum with some absorption lines.
Ionization
+
Hydrogen
_
+
_
++
Helium
_
_
++
"Ion"
Absorbing a high energy photon and atomic collisions can both lead to ionization.
_
_
Spectrum of the Sun• Complicated objects
=> many different elements
• Nearly continuous absorption spectrum
• What causes emission/absorption of light at certain wavelengths?
Why emission lines?
.
..
...
hot cloud of gas
- Photon absorption/atomic collisions excite atoms
- Electron drops back to lower level
- Photons at specific frequencies emitted
Stellar Spectra
Star
Fusion generates
continuous spectrum
Sun's 'atmosphere' absorbs specific wavelengths
Question How does the pitch or tone of a sound wave
change when the source of the sound is moving towards or away from you?
What about when you are moving towards or away from the source?
Does this effect occur for all types of waves or just for sound waves?
Doppler Shifted Atomic Spectra
• Why don’t we see the color of everyday objects change as they move?
• Star wobbling causes Doppler shift of its absorption lines.
• Only gives information about velocity along line of sight!