Chapter 5
The Nature of Light
Guiding Questions
1. How fast does light travel? How can this speed be measured?
2. Why do we think light is a wave? What kind of wave is it?
3. How is the light from an ordinary light bulb different from the
light emitted by a neon sign?
4. How can astronomers measure the temperatures of the Sun
and stars?
5. What is a photon? How does an understanding of photons
help explain why ultraviolet light causes sunburns?
6. How can astronomers tell what distant celestial objects are
made of?
7. What are atoms made of?
8. How does the structure of atoms explain what kind of light
those atoms can emit or absorb?
9. How can we tell if a star is approaching us or receding from
us?
Light travels through empty space
incredibly fast.
Italian Galileo unsuccessfully attempted to
measure the speed of light by asking an
assistant on a distant hilltop to open a lantern
the moment Galileo opened his lantern.
Light travels through empty space at a
speed of 300,00 km/s, called c
In 1676, Danish
astronomer Olaus Rømer
discovered that the exact
time of eclipses of
Jupiter’s moons varied
based on how near or far
Jupiter was to Earth.
This occurs because it
takes varying amounts of
time for light to travel the
varying distance between
Earth and Jupiter.
Improving measurements of c
In 1850, Frenchmen Fizeau and Foucalt showed that light takes a
short, but measurable, time to travel by bouncing it off a rotating
mirror. The light returns to its source at a slightly different position
because the mirror has moved during the time light was traveling a
known distance.
Light is electromagnetic radiation. It has
a wavelength l and a frequency n.
White light is composed of all colors which can
be separated into a rainbow, or a spectrum, by
passing the light through a prism.
Visible light has a wavelength ranging from 400
nm (blue) to 700 nm (red).
Although Isaac Newton suggested that light was made
of tiny particles called PHOTONS 130 years earlier,
Thomas Young demonstrated in 1801 that light has
wave-like properties. He passed a beam of light
through two narrow slits which resulted in a pattern of
bright and dark bands on a stream.
This is the
pattern
one would
expect if
light had
wave-like
properties.
Imagine water passing through two narrow openings as
shown below. As the water moves out, the resulting
waves alternatively cancel and reinforce each other,
much like what was observed in Young’s double slit
experiment.
This is the
pattern
one would
expect if
light had
wave-like
properties.
It turns out that light has characteristics of both particles and
waves. Light behaves according to the same equations that govern
electric and magnetic fields that move at the speed c, as predicted
by Maxwell and verified by Hertz.
Light is also called electromagnetic radiation,
Electromagnetic radiation consists of oscillating electric and
magnetic fields. The distance between two successive wave
crests is called the wavelength and is designated by the letter l.
Stars produce
electromagnetic radiation in a
wide variety of wavelengths
in addition to visible light.
Astronomers sometimes
describe EM radiation in
terms of frequency, n, instead
of wavelength, l. The
relationship is:
Speed = distance/time
c=ln
Where c is the speed of light, 3 x 108 m/s
A dense object emits electromagnetic
radiation according to its temperature.
WIEN’S LAW: The peak
wavelength emitted is
inversely proportional to
the temperature.
In other words, the
higher the temperature,
the shorter the
wavelength (bluer) of
the light emitted.
BLACKBODY CURVES: Each of these curves
shows the intensity of light emitted at every
wavelength for idealized glowing objects (called
“blackbodies”) at three different temperatures.
Note that for the hottest
blackbody, the maximum
intensity is at the shorter
wavelengths and the total
amount of energy emitted
is greatest.
Astronomers most often use the Kelvin or Celsius
temperature scales.
In the Kelvin scale, the
0 K point is the
temperature at which
there would be no
atomic motion. This
unattainable point is
called absolute zero.
In the Celsius scale,
absolute zero is –273º
C and on the
Fahrenheit scale, this
point is -460ºF.
The Sun is nearly a blackbody.
Wien’s law and the Stefan-Boltzmann let
us discover the temperature and intrinsic
brightness of stars from their colors.
Wien’s law relates wavelength of maximum
emission for a particular temperature:
lmax = 0.0029 Tkelvins
Stefan-Boltzmann law relates a star’s energy
output, called ENERGY FLUX, to its temperature
ENERGY FLUX = sT4 = intensity =Power/Area
ENERGY FLUX is measured in joules per second per square meter of a
surface, and the constant s = 5.67 X 10-8 W m-2 K-4
Energy of a photon in terms of wavelength:
E=hc/l
where h = 6.625 X 10-34 J s
or h = 4.135 X 10-15 eV
h = Planck’s constant
Energy of a photon in terms of frequency:
E = h n where n is the frequency of light
High energy light has short wavelength and high
frequency.
Each chemical element produces its own
unique set of spectral lines.
The brightness of spectral lines depend
on conditions in the spectrum’s source.
Continuum = rainbow of light
Law 1 A hot opaque body,
such as a perfect blackbody,
or a hot, dense gas produces
a continuous spectrum -- a
complete rainbow of colors
with without any specific
spectral lines. (This is a
black body spectrum.)
Emission lines due electron relaxation
Law 2 A hot, transparent gas produces an
emission line spectrum - a series of bright
spectral lines against a dark background.
Absorption lines due to electron excitation
Law 3 A cool, transparent gas in front of a
source of a continuous spectrum produces
an absorption line spectrum - a series of
dark spectral lines among the colors of the
continuous spectrum.
Kirchhoff’s Laws
Here is the Sun’s spectrum,
viewed with a prism or diffraction grating.
But, where does light actually
come from?
Light comes from the
movement of electrons in
atoms.
Rutherford’s experiment revealed the nature of atoms
Alpha particles from a radioactive source are channeled through a very thin
sheet of gold foil. Most pass through, showing that atoms are mostly empty
space, but a few bounce back, showing the tiny nucleus is very massive.
An atom
consists of a
small, dense
nucleus
surrounded by
electrons
Nucleus = protons + neutrons
• The nucleus is bound by the strong force.
• All atoms with the same number of protons
have the same name (called an element).
• Atoms with varying numbers of neutrons
are called isotopes.
• Atoms with a varying
numbers of electrons
are called ions.
Spectral lines are produced when an
electron jumps from one energy
level to another within an atom.
Bohr’s formula for hydrogen lines
DE = hc/l
= E0 [ 1/N2 – 1/n2 ]
N = number of inner orbit
n = number of outer orbit
R = Rydberg constant (1.097 X 107 m-1)
l= wavelength of emitted or absorbed
photon
1/l = R [ 1/N2 – 1/n2 ]
The wavelength of a spectral line is affected
by the relative motion between the source and
the observer.
Doppler Shifts
• Red Shift: The observer and source are separating, so
light waves arrive less frequently.
• Blue Shift: The observer and source are approaching,
so light waves arrive more frequently.
Dl/lo = v/c
Dl = wavelength shift
lo = wavelength if source is not moving
v = speed of source
c = speed of light
What can we learn by
analyzing starlight?
• A star’s temperature
– by peak wavelength
• A star’s chemical composition
– by spectral analysis
• A star’s radial velocity
– from Doppler shifts
Guiding Questions
1. How fast does light travel? How can this speed be measured?
2. Why do we think light is a wave? What kind of wave is it?
3. How is the light from an ordinary light bulb different from the
light emitted by a neon sign?
4. How can astronomers measure the temperatures of the Sun
and stars?
5. What is a photon? How does an understanding of photons
help explain why ultraviolet light causes sunburns?
6. How can astronomers tell what distant celestial objects are
made of?
7. What are atoms made of?
8. How does the structure of atoms explain what kind of light
those atoms can emit or absorb?
9. How can we tell if a star is approaching us or receding from
us?