Transcript Document

Light
Radiation and Spectra
Chapter 5
What is Light?
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• 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
QuickTime™ and a
TIFF(Uncompres sed) dec ompressor
are needed to see thi s pic ture.
Waves
• Wavelength ( l )
– 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 = lf
• Wavelength inversely related to frequency
l=c/f
– high frequency = short wavelength
– low frequency = long wavelength
Particles as Waves
• “Wave Packet”
– particle/photon = localized wave
Properties of Light
•
Color
–
Depends on frequency
•
•
•
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
blue = high frequency = short wavelength
red = low frequency = long wavelength
Carries energy (heat)
–
Photon energy
h = Planck’s constant
E=hf
• high frequency = high energy = blue
• low frequency = low energy = red
Red light has ____ than blue light.
A. larger frequency, energy, and wavelength
B. smaller frequency, energy, and wavelength
C. larger frequency and energy, but smaller wavelength
D. smaller frequency and energy, but larger wavelength
Which of the following travels fastest?
A.
B.
C.
D.
E.
}
radio waves
infrared (heat) waves
microwaves
blue light waves
none of the above
All are
types of
light!
All types of light travel at the same speed the “speed of light”, c
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 from
you, and an identical 100-watt light bulb is placed
100 feet away from you, which will appear brighter?
A. The closer one
B. The farther one
C. They will appear the same brightness
How much fainter will the far one appear
compared to the close one?
A.
B.
C.
D.
Twice as faint
10 times fainter
100 times fainter
1000 times fainter
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
~ 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
Qui ckTime™ and a TIFF ( Uncompressed) decompressor are needed to see this pictur e.
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
Blackbody Radiation Laws
• Luminosity, L
L = energy emitted per second
• Luminosity for a spherical object (a star)
Stefan-Boltzmann Law
L = 4p R2 s T4
R = radius (size) of star; T = temperature
– double size, luminosity increases by 2x2 = 4
– double temperature; luminosity increases by
2x2x2x2 = 16
WORKBOOK EXERCISE:
Luminosity Temperature and Size
Part I
(pages 33-35 in workbook)
Blackbody Radiation
Blackbody Radiation Laws
• Color
– Wavelength where most light emitted
Wien’s Law
lmax = 3 x 106 / T
T in Kelvin; lmax in nanometers (1 nm=10-9m)
• Cool stars are red
• Hot stars are blue
– Color indicates temperature!
Qu ickT ime™ an d a T IFF (Unc omp ress ed) d ecom pre ssor are n eed ed to see this pi
As T
As T
, Wavelength
, Wavelength
, Color = redder
, Color = bluer
Homework: WORKBOOK EXERCISE
Blackbody Radiation
(pages 37-40 in workbook)
The graph above shows blackbody spectra for three different
stars. Which of the stars is at the highest temperature?
A. Star A
Because peak energy emission
B. Star B
occurs at shortest wavelength
C. Star C
Qu i c k Ti m e ™ a n d a TI FF (U n c o m p re s s e d ) d e c o m p re s s o r a r e
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)
Doppler Shift
Redshift
Blueshift
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)
Solar
Spectrum
N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
Spectroscopy
– Emission Line spectrum
• Only certain colors are present (discrete lines)
• Spectrum for each element unique (like fingerprints)
Model Atom
• Nucleus
– contains protons and neutrons
– number of protons = element
hydrogen
(1 proton = hydrogen, 2 protons = helium, etc.)
– number of neutrons about same as
protons
• Isotope = different number of neutrons
Isotopes of hydrogen
helium
Model Atom
• Electrons orbit nucleus
– Number of electrons = number of protons
• Ionization = removing electrons
– Only certain orbits are allowed
hydrogen
helium
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)
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
Qui ckTi me™ and a TIFF ( Uncompressed) decompressor are needed to see thi s picture.
• Energy Levels
– Different for each element
• each element has unique set of absorption/emission
lines
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/energies
WORKBOOK EXERCISE:
“Types of Spectra”
(pages 41-42 in workbook)
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 spectrum
B. bright line emission spectrum
C. 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 spectrum
B. bright line emission spectrum
C. continuous spectrum