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Atom and Light
ASTR 14: Exploring the Universe
Outline
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Nature of Light
Basic Properties of Light
Atomic Structure
Periodic Table of the Elements
Three Types of Spectra
Interaction of Light and Matter
Useful Application – Doppler Effect
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Nature of Light
• What is the “Nature” of Light?
• Observed Properties of Light
– reflection, refraction, diffraction, & interference
• Wave Properties
– reflection, refraction, diffraction, & interference
• Particle Properties
– reflection, refraction
• Particle-Wave Duality
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Wave Properties of Light
• Wave Properties
– diffraction
– Interference
• Young’s Double-Slit
Experiment (1801)
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Wave Properties of Light
• Electromagnetic
Waves
– wavelength (meter)
– frequency (cycles per
second = Hertz = Hz)
• Speed of Light
– constant within a
medium
– absolute speed
– 3.0 x 108 m/s in
vacuum
Fizeau-Foucault Method (1850)
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Electromagnetic Spectrum
• Speed of Light =
wavelength x
frequency
• C=lxn
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Particle Properties of Light
• Max Planck (1900)
– Blackbody Radiation
– Electromagnetic energy
(radiation) is emitted in
discrete, particlelike
packet.
• Albert Einstein (1905)
– Re-interpret Planck’s result
– Photoelectric Effect
– Light as particles -photons
• Energy of a Photon
– The energy of a photon is
proportional to its
frequency.
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Blackbody Radiation
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Blackbody Radiation
• Wien’s Law
– The wavelength of
maximum emission of a
blackbody is inversely
proportional to its
temperature.
0.0029
lmax (m) 
T (K )
• Stefan-Boltzmann Law
(Box 5-2)
– The energy flux (power
per unit area) of a
blackbody is proportional to
the 4th power of its
temperature.
Flux(Wm 2 )  T 4 ;  5.67 x108Wm 2 K 4
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Atomic Structure
• Rutherford’s
Experiment (1910)
• Rutherford’s Model of
the Atom
• Atomic Structure
– cloud of electrons
– nucleus
• protons
• neutrons
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Atomic Structure
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Three Types of Spectra
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Interaction of Light and Matter
• Kirchoff’s 1st Law
– A hot opaque body, such as a perfect blackbody, or a hot, dense
gas produces a continuous spectrum.
• Kirchoff’s 2nd Law
– A hot, transparent gas produces an emission line spectrum.
• Kirchoff’s 3rd Law
– A cool, transparent gas in front of a source of a continuous
spectrum produces an absorption line spectrum. The
absorption lines in the absorption line spectrum of a particular
gas occur at exactly the same wavelengths as the emission lines
in the emission line spectrum of the same gas.
• Types of spectra of matter can produce is dictated by the
physical condition/state of the matter.
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Interaction of Light and Matter
• Bohr Model of Hydrogen Atom
– energy levels or states
• Quantum Rules
– Electron can only occupy at discrete energy level.
– Electron can jump from one energy level to another only if it
gains or loses a specific amount of energy equal to the
differences of the levels.
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Interaction of Light and Matter
Electron Transitions in Hydrogen
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Interaction of Light and Matter
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Interaction of Light and Matter
States of Matter
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An Example
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Doppler Effect
l = wavelength shift
l
v
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lo c
lo = lab wavelength
V = radial velocity
c = speed of light
+ = redshift
- = blueshift
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Christian Doppler (1842)
The apparent change in wavelength or frequency of radiation due to relative
motion between the source and the observer along the line-of-sight.
If the relative motion between the source and the observer is moving away
from each other, the observed spectral line is longer than the lab
wavelength, it is called a redshift. If the relative motion between the source
and the observer is moving towards each other, the observed wavelength is
shorter than the lab wavelength, it is called a blueshift.
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Doppler Effect
Stationary Source
Moving Source
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An Example
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