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Measuring Light Quantitatively
•Spectroscopy: measuring wavelengths () and frequencies () emitted or absorbed by matter; composition of stars
•Photometry: measuring the intensity of light; luminosity of stars
Measuring Light Quantitatively
•Polarimetry: measurement and interpretation of the polarization of light waves.
• Polarization: waves that have traveled through or have been reflected, refracted, or diffracted by some material; plane(s) of transmission absorbed
RadiationRadiationRadiant Energy: Radiant Energy: • Electromagnetic (EM) energyElectromagnetic (EM) energy• Energy that spreads out as it travels Energy that spreads out as it travels
from its sourcefrom its source• Follows an inverse square lawFollows an inverse square law• Can be measured in many different Can be measured in many different
waysways
Properties of Light
Light is radiant energyLight is radiant energy• Can travel through space Can travel through space
without a physical mediumwithout a physical medium• Speed = 300,000 km/secSpeed = 300,000 km/sec• Speed in a vacuum is constant Speed in a vacuum is constant
and is denoted by the letter “c”and is denoted by the letter “c”
Properties of Light
• c is reduced as it enters c is reduced as it enters transparent materials; transparent materials;
• The speed is dependent on color The speed is dependent on color (Blue light slows more than red)(Blue light slows more than red)
• Lenses and prisms work this wayLenses and prisms work this way• Light is a “mix” of electrical and Light is a “mix” of electrical and
magnetic energy magnetic energy
Nature of LightNature of Light
•Light shows properties of waves•Can measure wavelength (λ) and
frequency (υ)
Nature of LightNature of Light•Light also behaves like a Light also behaves like a
stream of particles called stream of particles called photonsphotons
•Each photon carries a specific Each photon carries a specific amount of energyamount of energy
•All particles can also behave as All particles can also behave as waveswaves
•Application: Photoelectric Application: Photoelectric effecteffect
•Mathematical relationships:Mathematical relationships:
c = c = = wavelength; = wavelength;
= frequency= frequency
c = speed of lightc = speed of light
As wavelength increases, As wavelength increases, frequency ____________frequency ____________
Energy and Light:Energy and Light:
E = hE = hE = energy; E = energy;
h = Planck’s constanth = Planck’s constant
As frequency increases, As frequency increases, energy ____________energy ____________
•The range of colors to which the human eye is sensitive is called the visible spectrumvisible spectrum
• Color is determined by
wavelengthwavelength ()• FrequencyFrequency (or (or ) ) is the number of
wave crests that pass a given point in 1 second (measured in Hertz, Hz)
•C Long C Long λλ; ; LowLow νν; ; Low E (Red)Low E (Red)•OO•L Mid L Mid λλ; ; MidMid νν; ; Mid E Mid E (Yellow)(Yellow)
•OO•R Short R Short λλ; ; HighHigh νν; ; High E High E (Violet)(Violet)
Electromagnetic Electromagnetic RadiationRadiation
• Wavelengths range: 10-14 m to 103 m
• Energy range follows the same pattern
• These trends make light a great probe for studying the Universe
• E-M spectrum includes radio, microwave, infrared, visible, ultraviolet, x-ray, and gamma radiations
Invisible Light in Our UniverseInvisible Light in Our Universe
www.warren-wilson.edu/.../sstephens/bragg2.html
Radio WavesRadio Waves• Produced in 1888 by
Hertz
• First cosmic detection - 1930’s
• Long wavelengths (big telescopes needed)
• Temperatures < 10 K
Very Large Array – New Mexico
M87 Galactic Center in radio
Radio WavesRadio WavesFor detection/study of:
Cosmic Background
Cold interstellar medium; site of star formation
Regions near neutron stars & white dwarfs
Dense regions of interstellar space (e.g. near the galactic center)
Milky Way in visible (top) and radio wavelengths
Infrared RadiationInfrared Radiation• Discovered by Sir Discovered by Sir
William Herschel William Herschel (around 1800)(around 1800)
• Long wavelength (Long wavelength (λλ); ); low frequency (low frequency (υυ))
• Temperature range: Temperature range: 10 -1010 -1033 K K
Spitzer Space Telescope
Infrared RadiationInfrared RadiationUseful in detecting:Useful in detecting:
• Cool stars
• Star Forming Regions
• Interstellar dust warmed by starlight
• Planets, Comets, Asteroids
M104 in visible light
M104 in IR
Ultraviolet Radiation• Discovered by J. Ritter
in 1801• Photographic plates
exposed by “light” beyond the violet
• Shorter λλ, higher , higher energyenergy
• Temperatures: 10Temperatures: 104 4 - - 101066 K K Hampton UV
Telescope
Ultraviolet RadiationUsed to detect/study:
• Supernova remnants
• Very hot stars
• Quasars
M101 in visible lightM101 in UV light
X-RaysX-Rays• Roentgen
discovered X rays in 1895
• First detected beyond the Earth in the Sun in late 1940s
• Used to study Neutron stars, Supernova remnants
Chandra x-ray telescope
The sun in x-ray
SPECTRASPECTRA
Kirchhoff’s LawKirchhoff’s Law
Continuous Spectrum:
•produced when dense, hot matter emits a continuous array of wavelengths
•we see it as white light
Emission Spectrum•when heated, a low-density
gas (low pressure) will emitemit light in specific wavelengths
• the spectrum produced is called a line spectrum (also called an emissionemission spectrum
Absorption Spectrum•Cool, low-density gas between the source and observer absorbs light of specific wavelengths
•one gas will absorb and emit in the same wavelengths
Hydrogen AtomHydrogen Atom
Light & The AtomLight & The Atom
• Electrons found in discrete Electrons found in discrete energy energy levelslevels
• Electrons absorb energy, move to Electrons absorb energy, move to higher levelshigher levels
• Electrons release energy as they Electrons release energy as they move to lower energy levelsmove to lower energy levels
Solar SpectrumSolar Spectrum
• The core of our star produces a The core of our star produces a continuous spectrumcontinuous spectrum
• Atoms in the atmosphere absorb Atoms in the atmosphere absorb the light the light
• These atoms emit light in random These atoms emit light in random directions – that produces dark directions – that produces dark lines in the spectrumlines in the spectrum
Solar SpectrumSolar Spectrum
Solar SpectrumSolar Spectrum• The dark lines are called
Fraunhofer linesFraunhofer lines
The Sun’s SpectrumThe Sun’s Spectrum
Arcturus SpectrumArcturus Spectrum
Thermal Radiation & Starlight
Wien’s Displacement LawWien’s Displacement Law• Heated bodies generally radiate across Heated bodies generally radiate across
the entire electromagnetic spectrumthe entire electromagnetic spectrum• There is one particular wavelength, There is one particular wavelength, λλmm, at , at
which the radiation is most intense and which the radiation is most intense and is given by Wien’s Law:is given by Wien’s Law:
λλmm = k/T = k/T
Where k is some constant and T is the Where k is some constant and T is the temperature of the bodytemperature of the body
Thermal Radiation & Starlight
• As the temperature of a star As the temperature of a star increases, the most intense increases, the most intense wavelengths become shorterwavelengths become shorter
• As an object heats, it appears to As an object heats, it appears to change color from red to white to change color from red to white to blueblue
Stefan-Boltzmann LawStefan-Boltzmann Law• As the temperature of a star
increases, the total energy output increases as the 4th power of the temperature
• E T4
•Motion-induced change in the observed wavelength of any wave — light or sound—is known as the Doppler effect
• If the source is moving toward the observer, waves become compressed
•A shorter wavelength will appear blueblue
•This is called a blue-shiftblue-shift
• If the source is moving away from the observer, waves will be “stretched out”
•A longer wavelength will appear redred
•Known as red-shiftred-shift
Absorption in the AtmosphereAbsorption in the Atmosphere
• Gases in the Earth’s atmosphere absorb Gases in the Earth’s atmosphere absorb electromagnetic radiation: most wavelengths from electromagnetic radiation: most wavelengths from space do not reach the groundspace do not reach the ground
• Visible light, most radio waves, and some infrared Visible light, most radio waves, and some infrared penetrate the atmosphere through penetrate the atmosphere through atmospheric atmospheric windowswindows, wavelength regions of high , wavelength regions of high transparencytransparency
• Lack of atmospheric windows at other Lack of atmospheric windows at other wavelengths is the reason for astronomers wavelengths is the reason for astronomers placing telescopes in spaceplacing telescopes in space
