Transcript Lecture 7

Lecture 7
ASTR 111 – Section 002
Outline
• Discuss Quiz 5
• Light
– Suggested reading: Chapter 5.1-5.2 and 5.65.8 of textbook
Light travels through empty space at a
speed of 300,000 km/s
• In 1676, Danish
astronomer
Olaus Rømer
discovered that
the exact time of
eclipses of
Jupiter’s moons
depended on
the distance of
Jupiter to Earth
• Light travels at 300,000 km/sec
• About how fast does your car travel in
km/hour when you are on the freeway?
• About how fast does your car travel in
km/sec when you are on the freeway?
• If it takes light 8 minutes to travel from the
sun to Earth, how long would it take you to
drive?
• Approximately what was the difference in
time of the eclipses that Olaus Rømer
observed?
Determining the Speed of Light
• Galileo tried
unsuccessfully to
determine the speed
of light using an
assistant with a
lantern on a distant
hilltop
• In 1850 Fizeau and Foucalt also experimented with light
by bouncing it off a rotating mirror and measuring time
• The light returned to its source at a slightly different
position because the mirror has moved during the time
light was traveling
• d=rt again gave c
Light is electromagnetic radiation
and is characterized by its wavelength ()
Frequency and Wavelength
 or f
1
time unit

length unit
The Greek letter “nu” and not the letter “v”
• Cars are traveling at 100 km/hr to the right
• What would you need to know in order be
able to tell how often a car will pass the
finish line?
Finish line
velocity = 100 km/hr
Replace cars with lines
Finish line
v = 100 km/hr
• Cars are traveling at 100 km/hr to the right
• What would you need to know in order be
able to tell how often a peak will pass the
finish line?
Finish line
v = 100 km/hr
c  
How often peak
passes finish line
Distance
between peaks
Finish line
How fast wave moves to right
Interference - destructive
A
B
Interference - destructive
C
D
Interference
A
?
B
Interference - simple
?
?
C
D
The electromagnetic spectrum
• Because of its
electric and
magnetic properties,
light is also called
electromagnetic
radiation
• Visible light falls in
the 400 to 700 nm
range
• Stars, galaxies and
other objects emit
light in all
wavelengths
• Which of the following has the highest
frequency?
– Visible light
– Radio waves
– Microwaves
– X-Rays
– Infrared light
– Ultraviolet light
– Gamma rays
• Which of the following has the highest
wavelength?
– Visible light
– Radio waves
– Microwaves
– X-Rays
– Infrared light
– Ultraviolet light
– Gamma rays
• Which of the following has the highest
speed?
– Visible light
– Radio waves
– Microwaves
– X-Rays
– Infrared light
– Ultraviolet light
– Gamma rays
• Which of the following has the highest
energy E (h is a constant)?
– Visible light
– Radio waves
– Microwaves
– X-Rays
– Infrared light
– Ultraviolet light
– Gamma rays
E
hc

The dual nature of light
• Particle
• Wave
Particle
What would you expect if instead of a laser
beam you used yellow spray paint beam?
Draw it!
Wave
The atom and light
Probing the atom
• An atom has a small dense nucleus composed
of protons (and neutrons)
• Rutherford’s experiments with alpha particles
shot at gold foil helped determine the structure
Spectral lines are produced when an electron jumps
from one energy level to another within an atom
• The nucleus of an atom is
surrounded by electrons that
occupy only certain orbits or
energy levels
• When an electron jumps from one
energy level to another, it emits or
absorbs a photon of appropriate
energy (and hence of a specific
wavelength).
• The spectral lines of a particular
element correspond to the various
electron transitions between
energy levels in atoms of that
element.
• Bohr’s model of the atom correctly
predicts the wavelengths of
hydrogen’s spectral lines.
Measurements in Astronomy
• In astronomy, we need to make remote
and indirect measurements
– Think of an example of a remote and indirect
measurement from everyday life
Using Light
• Light has many properties that we can use
to learn about what happens far away
• Light interacts with matter in a special way
• Only photons with
special wavelengths
will interact with atom
• How will this affect
what a person will see
at point X?
• When is the atom
“hotter”?
X
• Why is UV light usually blamed for skin
cancer? What is special about it
compared to other light sources?
What will
the
spectrum
look like
here?
Emission line spectrum
Continuous Spectrum
• A blackbody emits
photons with many
energies (wavelengths) –
a continuous spectrum
• A prism bends photons
more or less depending
on their wavelength
What will
the
spectrum
look like
here?
Absorption Spectrum
Absorption vs. Emission
• What type of spectrum is produced when the
light emitted from a hot, dense object passes
through a prism?
• What type of spectrum is produced when the
light emitted directly from a cloud of gas
passes through a prism?
• Describe the source of light and the path the
light must take to produce an absorption
spectrum
• There are dark lines in the absorption spectrum
that represent missing light. What happened to
this light that is missing in the absorption line
spectrum?
From Lecture Tutorials for Introductory Astronomy, page 61.
Each chemical element produces its
own unique set of spectral lines
• Stars like our Sun have low-density, gaseous atmospheres
surrounding their hot, dense cores. If you were looking at the
spectra of light coming from the Sun (or any star), which of the
three types of spectra would be observed?
• If a star existed that was only a hot dense core and did not
have a low-density atmosphere surrounding it, what type of
spectrum would you expect this particular star to give off?
• Two students are looking at a brightly lit full Moon, illuminated
by reflected light from the Sun. Consider the following
discussion between two students about what the spectrum of
moonlight would look like:
– I think moonlight is just reflected sunlight, so we will see the Sun’s
absorption line spectrum.
– I disagree, an absorption spectrum has to come from a hot, dense
object. Since thie Moon is not a hot, dense object, it can’t give off an
absorption line spectrum.
Do you agree or disagree with either or both of these students? Explain
your reasoning.
• Imagine that your are looking at two different spectra
of the Sun. Spectrum #1 is obtained using a
telescope that is in a high orbit far above Earth’s
atmosphere. Spectrum #2 is obtained using a
telescope located on the surface of Earth. Label
each spectrum below as either Spectrum #1 or
Spectrum #2.
• Would this make sense?
This dark line was removed
Energy and electromagnetic
radiation
Planck’s law relates the
energy of a photon to its
frequency or wavelength
E
hc

E = energy of a photon
h = Planck’s constant
c = speed of light
 = wavelength of light
The value of the constant h
in this equation, called
Planck’s constant, has been
shown in laboratory
experiments to be
h = 6.625 x 10–34 J s
Three Temperature Scales
Color and Temperature
An opaque object emits electromagnetic radiation
according to its temperature
Blue: Hot or Not?
http://www.straightdope.com/mailbag/mhotflame.html