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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
Radiation
Radiant Energy:
• Electromagnetic (EM) energy
• Energy that spreads out as it travels
from its source
• Follows an inverse square law
• Can be measured in many different
ways
Properties of Light
Light is radiant energy
• Can travel through space without a
physical medium
• Speed = 300,000 km/sec
• Speed in a vacuum is constant and is
denoted by the letter “c”
Properties of Light
• c is reduced as it enters transparent
materials;
• The speed is dependent on color (Blue
light slows more than red)
• Lenses and prisms work this way
• Light is a “mix” of electrical and
magnetic energy
Nature of Light
• Light shows properties of waves
• Can measure wavelength (λ) and
frequency (υ)
Nature of Light
• Light also behaves like a stream
of particles called photons
• Each photon carries a specific
amount of energy
• All particles can also behave as
waves
• Application: Photoelectric effect
• Mathematical relationships:
c = 
 = wavelength;
 = frequency
c = speed of light
As wavelength increases,
frequency ____________
Energy and Light:
E = h
E = energy;
h = Planck’s constant
As frequency increases, energy
____________
• The range of colors to which the
human eye is sensitive is called the
visible spectrum
• Color is determined by wavelength
()
• Frequency (or ) is the number of
wave crests that pass a given point in
1 second (measured in Hertz, Hz)
• C Long λ; Low ν; Low E (Red)
•O
• L Mid λ; Mid ν; Mid E (Yellow)
•O
• R Short λ; High ν; High E
(Violet)
Electromagnetic Radiation
• 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 Universe
www.warren-wilson.edu/.../sstephens/bragg2.html
Radio Waves
• Produced in 1888 by
Hertz
• First cosmic detection
- 1930’s
• Long wavelengths (big
telescopes needed)
• Temperatures < 10 K
M87 Galactic Center
in radio
Very Large Array – New
Mexico
Radio Waves
For 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 Radiation
• Discovered by Sir
William Herschel
(around 1800)
• Long wavelength (λ);
low frequency (υ)
• Temperature range:
10 -103 K
Spitzer Space
Telescope
Infrared Radiation
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 λ, higher
energy
• Temperatures: 104 106 K
Hampton UV
Telescope
Ultraviolet Radiation
Used to detect/study:
• Supernova remnants
• Very hot stars
• Quasars
M101 in visible
UV
light
light
X-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
The
sun in x-ray
telescope
SPECTRA
Kirchhoff’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 emit light in
specific wavelengths
• the spectrum produced is called a
line spectrum (also called an
emission spectrum
Emission
spectrum of H
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
Spectra
and Stars
Hydrogen Atom
Light & The Atom
• Electrons found in discrete energy
levels
• Electrons absorb energy, move to
higher levels
• Electrons release energy as they
move to lower energy levels
Solar Spectrum
• The core of our star produces a
continuous spectrum
• Atoms in the atmosphere absorb
the light
• These atoms emit light in random
directions – that produces dark
lines in the spectrum
Solar Spectrum
Solar Spectrum
• The dark lines are called
Fraunhofer lines
The Sun’s Spectrum
Arcturus Spectrum
Thermal Radiation &
Starlight
Wien’s Displacement Law
• Heated bodies generally radiate across
the entire electromagnetic spectrum
• There is one particular wavelength, λm,
at which the radiation is most intense
and is given by Wien’s Law:
λm = k/T
Where k is some constant and T is the
temperature of the body
Thermal Radiation &
Starlight
• As the temperature of a star
increases, the most intense
wavelengths become shorter
• As an object heats, it appears to
change color from red to white to
blue
Spectroscopy
Stefan-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 blue
• This is called a blue-shift
• If the source is moving away from
the observer, waves will be
“stretched out”
• A longer wavelength will appear red
• Known as red-shift
Absorption in the Atmosphere
• Gases in the Earth’s atmosphere absorb
electromagnetic radiation: most wavelengths from
space do not reach the ground
• Visible light, most radio waves, and some infrared
penetrate the atmosphere through atmospheric
windows, wavelength regions of high
transparency
• Lack of atmospheric windows at other
wavelengths is the reason for astronomers
placing telescopes in space