Chapter 5
Basic 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.
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. :
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, λ.
Light’s 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.
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)
Interference of light by a double slit.
Interference of water waves by a double slit.
Dispersion of light by a prism according to
wavelength λ or frequency ν.
Low energy radiation (longest λ, shortest ν) is
dispersed the least, high energy radiation
(shortest λ, longest ν) the most.
The electromagnetic spectrum (schematic).
Kirchhoff’s Laws.
Kirchhoff’s Laws illustrated.
The spectrum of the Sun is an absorption
spectrum, i.e. a hot gas viewed against a brighter
continuum source.
A high resolution view. Most of the dark spectral
lines are caused by atoms of gaseous iron in the
Sun’s atmosphere.
An absorption-line spectrum made into a spectral
intensity tracing.
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.
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 . Wien’s law:
0.0029
max in meters  
T
The brightness of a distant object decreases in
proportion to the inverse square of its distance.
According to Einstein’s famous relationship:
E  m c
2
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.
A plexiglass simulation of a gravitational lens created by
Charles Dyer and Robert Roeder (1981).
Double quasar, QSO 0957+561. The image of a single
background quasar split into two halves by an
intervening galaxy.
Huchra’s Lens, quasar Q2237+030 lensed by
galaxy ZW 2237+030.
Gravitational lensing in the galaxy cluster Abell 370.
A close-up view.
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.



observed  rest  v


rest
c
Interference of light by a straight edge.
The atomic and ionic lines seen in stellar spectra provide
a measure of a star’s temperature. Helium lines denote
hot stars (20,000K), molecular bands cool stars (3000K)
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.
Spectral types track the
differences between stars.
temperature
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)
Astronomical Terminology
Temperature. A measure of the thermal energy of an
object.
Luminosity. A measure of an object’s 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.
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.
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.
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:
f
262Hz  440Hz
Speed 
 332m/s 
 332m/s   226m/s
f
262Hz