Chapter 5 Basic Properties of Light

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Chapter 5Basic Properties of Light

• The chapter on Light is important for all studies
  in astronomy. Thus, it is important to review
  those properties in ASTR 1100.
• 1. Examine the nature of light as a wave and
  particle phenomenon in terms of its ability to
  carry packets of energy.
• 2. Note the various types of information ? line
  of sight speed, radiation temperature, chemical
  composition, etc. ? that can be established from
  the study of light from planets, stars, galaxies,
  gas clouds, etc.

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Light is a phenomenon that is characterized by
photons, which have both wave characteristics and
particle characteristics. Because light has the
properties of both a particle (the photoelectric
effect) and a wave (narrow slit interference), it
is usually pictured as a sine wave with an arrow
attached, e.g.
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Light is a form of electromagnetic
radiation since it carries both a varying
electric field and a varying magnetic field at
right angles to each other. It is characterized
by its wavelength, ?.
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Lights Particle-Like Nature is demonstrated
by The Photoelectric Effect. Light incident on
alkali metals and other solids (potassium,
bismuth, calcium, antimony, etc.) is able to
release electrons from the surface, if they are
energetic enough.
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Wave Nature of Light The wave nature of light
is demonstrated by the interference effects in
straight edge diffraction, single slit
diffraction, and double slit diffraction
experiments using a point source with either
monochromatic or white light. Straight edge
diffraction Single slit diffraction (top
monochromatic, bottom white light) Double slit
diffraction (top monochromatic, bottom white
light)
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Interference of light by a double
slit.
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Interference of water waves by a
double slit.
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Dispersion of light by a prism
according to wavelength ? or frequency ?.
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Low energy radiation (longest ?,
shortest ?) is dispersed the least, high energy
radiation (shortest ?, longest ?) the most.

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• The electromagnetic spectrum (schematic).

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Kirchhoffs Laws.
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Kirchhoffs Laws illustrated.
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The spectrum of the Sun is an absorption
spectrum, i.e. a hot gas viewed against a
brighter continuum source.
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A high resolution view. Most of the
dark spectral lines are caused by atoms of
gaseous iron in the Suns atmosphere.
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An absorption-line spectrum made into
a spectral intensity tracing.
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How the colour of an object corresponds
to its temperature. Initially hot objects glow
red, then yellow-orange, and finally white, i.e.
white hot, as the temperature increases. The
resulting radiation is referred to as black body
radiation.
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Properties of black body radiation 1. Energy
(light) is emitted at all wavelengths, except for
? 0 and ? ?. 2. The form of the continuous
energy distribution is given by the Planck
function. 3. As temperature T increases, the
energy output increases at all wavelengths ?. 4.
As T increases, the energy output increases most
rapidly for small ?. Wiens law
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The brightness of a distant object
decreases in proportion to the inverse square of
its distance.
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According to Einsteins famous
relationship For light Thus, since light
carries energy, photons can be considered to have
mass as they travel at the speed of light. At
rest they are massless. And, since mass is
affected by gravity, light rays can be deflected
when passing near massive objects stars, massive
galaxies, clusters of galaxies, etc.
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A plexiglass simulation of a
gravitational lens created by Charles Dyer and
Robert Roeder (1981).
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Double quasar, QSO 0957561. The image of a
single background quasar split into two halves by
an intervening galaxy.
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Huchras Lens, quasar Q2237030
lensed by galaxy ZW 2237030.
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Gravitational lensing in the galaxy
cluster Abell 370.
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A close-up view.
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Another property of light arising from its wave
properties is that it is subject to the Doppler
Effect. The wavelengths of photons from distant
objects moving relative to the observer appear to
be either stretched to longer wavelengths or
compressed to shorter wavelengths when viewed
by us, if they are moving either away from us
(red shift) or towards us (blue shift ),
respectively. The Doppler Effect was first noted
for sound waves, but applies to light waves as
well by extension.
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Interference of light by a straight
edge.
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The atomic and ionic lines seen in
stellar spectra provide a measure of a stars
temperature. Helium lines denote hot stars
(20,000K), molecular bands cool stars (3000K)
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• The spectral lines in the spectra of stars are
  from atomic species in the gas of stellar
  atmospheres that produce electronic transitions
  in the specific temperature ranges applicable to
  stars.

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• Spectral types track the temperature differences
  between stars.

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Hot stars put out a lot more light energy than
cool stars, since radiance varies as T4.
(Stefan-Boltzmann Law) The spectral sequence for
stars O B A F G K M (R N S) is a temperature
sequence. O stars are hottest, M stars
coolest. Oven Baked Ants, Fried Gently, Kept
Moist, Retain Natural Succulence Oh, Be A Fine
Girl/Guy, Kiss Me (Right Now, Smack)
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Astronomical Terminology Temperature. A measure
of the thermal energy of an object. Luminosity. A
measure of an objects absolute brightness in
terms of its radiation output. Continuous
Spectrum. A spectrum consisting of an unbroken
band of colour from the ultraviolet to infrared
regions. Absorption Spectrum. A continuous
spectrum interlaced by dark lines and
bands. Emission Spectrum. A spectrum consisting
only of bright lines or bands. Electromagnetic
Radiation. The generic description of light
ranging from gamma-rays to radio waves. Doppler
Effect. The change in frequency of light or sound
caused by radial motion of the source relative to
the observer.
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Sample Questions

• 1. We know that the speed of light in a vacuum is
  300,000 km/s. Is it possible for light to travel
  at a lower speed? Explain your answer.
• Answer Yes, light travels more slowly in
  different media, for example air, water or glass,
  according to the index of refraction for the
  medium.

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• 2. An object somewhere near you is emitting a
  pure tone at middle C on the octave scale, i.e.
  at a frequency of 262 Hz. You, having perfect
  pitch, hear the tone as A above middle C on the
  octave, which is at a frequency of 440 Hz.
  Describe the motion of that object relative to
  where you are standing.

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• Answer. The object is moving toward you at a
  speed of 226 m/s. Because the note is heard at a
  higher frequency, the sound waves reach you at a
  shorter wavelength than that at which they were
  generated by the object. Wavelength and frequency
  are inversely proportional to one another. The
  sound waves are therefore blue shifted (appear
  at shorter wavelengths than originally), so the
  object must be travelling toward you.
• The actual speed of the object is found from
  arithmetic