Transcript September 24

Exam 1 Next Tuesday
Covers readings from Chapters 1-5 and S1.
Two Essay questions
25 multiple choice questions.
Allowed one sheet of notes (standard sized)
with writing on one side only
Discussion
Will the Moon still have phases? Explain.
Discussion
What would happen to the Moon if its
orbital period was less than the Earth’s day?
Newton and Kepler
Newton’s laws of motion combined with
Kepler’s laws led directly to the formulation of
the universal law of gravity.
Using Newton’s laws and the law a gravity you
can derive Kepler’s laws of planetary motion.
Planetary motion is thus just a special case of
ordinary Newtonian motion.
Kepler’s 1st law
The planets move in ellipses with the Sun
at one focus.
Conic section orbits are a natural outcome
of the 1/d2 nature of the gravitational
force.
Kepler’s 2nd law
A line drawn from the planet to the Sun
sweeps out equal areas in equal intervals
of time.
Just another way of saying angular
momentum is conserved which comes
from Newton’s 2nd law of motion.
Kepler’s 3rd law
The square of the sidereal period is
proportional to the cube of the semimajor
axis of the orbit: p2 = a3
Newton’s laws require:
4
3
p 
a
G(M1  M 2 )
2
2
The Scientific method at work
• Use the simplest model to explain the
observations
• Refine the model, make it more
complicated, only if new observations
require
• Explain why the model works
Explanation vs. usefulness
Kepler’s laws were adopted because they
were useful for predicting planetary
motions.
Newton’s laws explained why Kepler’s laws
worked in a broader context.
Discovery of Neptune
Saturn and Uranus did not follow Kepler’s laws
exactly
Le Verrier and Adams use these deviations to
predict the existence and position of Neptune
which was observed in 1846
Gravity
That one body may act upon another at a
distance through a vacuum, … and force may
be conveyed from one to another, is to me so
great an absurdity, that I believe no man who
has … a competent faculty in thinking, can
ever fall into it.
Isaac Newton 1692
Light
Light
The primary source of information we have about
the solar system comes from the light received
from space.
Does light travel instantaneously
from one place to another?
Galileo unsuccessfully tried to measure
speed of light between two mountains using
lanterns.
Speed of Light
Olaus Romer – in 1676 noticed that the timing of
eclipses of the moons of Jupiter was dependant
on the relative positions of Jupiter and the Earth.
The eclipse of a moon of Jupiter was 16.6 minutes
later when Jupiter was near conjunction than
when Jupiter was near opposition.
Discussion
Why would the position of the Jupiter in the
sky cause a change in the timing of the
eclipses of the moons of Jupiter?
Romer’s Speed of Light Determination
Fizeau-Foulcoult Apparatus
Light travels at a speed of 3.00 × 108 m/s
What is the Nature of Light?
Light is a form of electromagnetic radiation,
i.e. light is oscillating electric and magnetic
fields.
An oscillating electric field produces an
oscillating magnetic field and an oscillating
magnetic field produces an oscillating electric
field.
Electric and Magnetic Waves
Water Waves
Frequency Wavelength Relation
 f c
  Wavelength
f  frequency
c  speed of the wave
Fields
Electric fields cause charged particles to move.
Magnetic fields cause magnets to move.
Electric Field Waves
Frequency and wavelength
Because the speed of light is constant, for
all observers and all frequencies, higher
frequency electromagnetic waves have
shorter wavelengths and lower frequency
electromagnetic waves of longer
wavelengths.
We see variations in wavelength or
frequency as color
White light contains all colors of the rainbow
The energy of electromagnetic
waves.
E = h  f = h  c/
h is a constant, f is the frequency
 is the wavelength, and c is the speed of light
The higher the frequency, or the smaller the
wavelength, the higher the energy.
The Electromagnetic Spectrum
Discussion
In the movie Superman, the caped wonder
claims to be able to tell the color of Lois Lane’s
underwear with his X-ray vision. Is this
possible, or did Superman rifle though her
dresser drawers when she wasn’t home?
Light interacts with matter in four
general ways
1.
2.
3.
4.
Emission
Absorption
Transmission
Reflection
Come up with an everyday example of each.
Three types of spectra
Continuous – emits all frequencies of colors
of light
Emission line – only specific frequencies are
emitted.
Absorption line – mostly a continuous
spectrum with specific frequencies missing
Continuous, Absorption and
Emission Spectra
Discussion
What is light?
Discussion
How can I create light with a magnet?
There are two types of emission
• Blackbody or thermal emission which
produces a continuous spectrum, i.e. all
colors of the rainbow.
• Line emission which produces only certain
wavelengths of light.
Blackbody radiation
In general the spectrum emitted by a hot object
depends on the composition of the object. But
there is a class of objects, called blackbodies
because they appear black, that are perfect
absorbers and perfect emitters. The spectrum
of a blackbody depends only on its
temperature.
Discussion
It is fairly easy to see that a perfectly black
object is a perfect absorber of light. But
why is a perfect absorber also a perfect
emitter? What do you think would happen
if a perfect absorber were not a perfect
emitter?
Discussion
All objects emit thermal (blackbody) radiation.
Why?
Atom
Temperature
Properties of thermal radiation
1) Blackbodies emit radiation at all
frequencies.
Thus, they emit a continuous spectrum, all the
colors of the rainbow.
Blackbody Radiation
Properties of thermal radiation
2) Wien’s law – the temperature of the object
is directly proportional to the frequency of
maximum emission.
Discussion
Why do you think the frequency of maximum
emission increases with the temperature of
the object?
Properties of thermal radiation
3) Stefan-Boltzmann law – the intensity of the
emission is proportional to the 4th power of the
temperature.
Thermal Spectra
Infrared Image
Ionized
hydrogen
gas cloud
Discussion
An excited electron will drop rapidly back
down to ground state, emitting a photon. If the
photon absorbed is the same wavelength as the
photon that is emitted, why do we see dark
absorption line features in the spectrum?