The Nature of Light
Electromagnetic radiation
emitted by astronomical objects
electromagnetic waves ® transfer of energy by a disturbance
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Young’s experiment ® wave nature |
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l = wavelength = length between crests/valleys |
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n = frequency = number of crests passing
per unit time |
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= cycles per sec (Hz)
= 1/P |
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P = period = time for wave to repeat
itself |
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nl = c (speed of
light - celeritas) = 3 x 1010
cm/sec |
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= 300, 000 km/s |
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visible light: 4000 < l > 8000 Ångstrom (1Å
= 10-8 cm) |
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For a star, let B= brightness =
power/area = power/4pd2 |
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If L = total output of star, L = Bx4pd2 and B =
L/4pd2 |
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Waves and charged
particles
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charged particles (electrons, protons)
and carry electric field, strength of field µ 1/d2 |
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collision or heating ® particle
vibration ® change in field ® change in electrical forces on other particles |
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change in electrical forces ® information
about original particle |
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®information transmitted through a disturbance (change in electrical
field) |
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magnetic field associated with
electrical field |
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®electromagnetic waves |
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Electromagnetic waves from moving
charged particles |
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in astronomical objects |
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propagate at speed of light, c (need
not be visible) |
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Slide 5
Two main processes for
e-m radiation
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Thermal radiation |
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constituent particles in constant
random motion ® e-m radiation |
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temperature @ amount of motion |
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radiation emitted over range of
frequencies |
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Spectral line radiation |
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discrete quantum mechanical line
radiation -- only at n corresponding
to quantized energy level differences |
Blackbody Radiation
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Blackbody absorbs (& re-emits) all
energy falling onto it |
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characteristic of any opaque surface
(e.g. stove burner |
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depends only on T of surface (not shape
or composition) |
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Kirkoff’s Law : perfect absorber =
perfect emitter = Blackbody |
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Intensity of radiation versus
frequency ® blackbody curve |
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Blackbody (Planck) curves
at different temperatures
Radiation laws – apply to
astronomical sources
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Wien’s law : |
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lpeak = 0.289 / T(K) cm |
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lpeak ® color: hotter ® bluer (shorter l) |
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cooler ® redder (longer l) |
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Greater total energy (summed over all
wavelengths) radiated by hotter objects |
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¯ |
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Stefan’s Law : |
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energy radiated/unit area of B-B
surface/unit time µ (temp)4 |
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F = sT4 |
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Flux = 5.67x 10-8T4
watt/m2/K4 |
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Stefan’s constant = s |
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Slide 10
e-m radiation ® stellar properties
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brightness ® energy output B = L/4pd2 |
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(if distance to object/star is known) |
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color ® surface temperature (T) of star |
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(blackbody) |
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lpeak = 0.289 / T(K) |
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how do we measure stellar distance? |
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Measuring stellar
distances
Slide 13
Slide 14
Slide 15
Slide 16
e-m radiation: spectral
lines
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Quantum transitions |
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energy
levels within atomic and molecular systems quantized |
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only discrete orbits/energies permitted |
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when atom makes a transition between 2
states, gives up energy corresponding
to the difference |
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discrete amounts of energy only
(photons) |
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types of transition |
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collisional excitation/deexcitation |
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radiative absorption + spontaneous emission; stimulated emission |
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Photon energy µ radiation frequency (color) |
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DE = Efinal - Einitial
= hn
(radiation) or ½mv2 (collisional) |
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Slide 18
Slide 19
Slide 20
Kirchhoff’s Laws
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Relate continuous spectra, emission
line spectra, & |
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absorption line
spectra |
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Luminous solid (dense gas) emits light
of all wavelengths ® continuous spectrum of radiation |
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Low density hot gas emits spectrum of
bright emission lines ® composition of gas |
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Low density cool gas absorbs
wavelengths from continuous spectrum ® dark absorption lines on continuous spectrum
(wavelengths same as emission lines from hot gas) |
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Slide 22