Transcript Chapter 4 Notes

CHAPTER 4:
Visible Light
and Other
Electromagnetic
Radiation
Blackbody



Object that absorbs all of
the radiation that it
receives (that is, it does
not reflect any light, nor
does it allow any light to
pass through it and out
the other side).
The energy that the
blackbody absorbs heats
it up, and then it will emit
its own radiation.
Emitted radiation is only
dependent on
temperature of the object
No object that is an ideal
blackbody, but many objects
behave approximately like
blackbodies..
Blackbody Radiation Curve
(Planck Curve)



Shows energy
emitted by a
blackbody
Only depends on
temperature
represented by the
area under the curve
Peak wavelength is
inversely proportional
to the temperature
Stellar surfaces emit light that is close to an ideal blackbody. We
can estimate the surface temperature of a star by examining the
intensity of emitted light across a wide range of wavelengths.
Wein’s Law


Formula that allows
us to determine the
temperature of a star.
It is based on the fact
that hotter objects
have more energy
than cooler objects
T = 2.987 x 107 KǺ
λ max
T = Temp in Angstroms; λ max = wavelength of peak temp
Stefan-Boltzman Law

Total energy being
emitted at all
wavelengths by the
blackbody
E = σ T4
σ= 5.67 x 10-8 w/m2K4
T = temp in Kelvin
E = energy in w/m2
A spectroscope is used to examine the
wavelengths of light emitted from a source
When a chemical is
burned, the light
produced is made of
only specific
wavelengths.
Different chemical
elements have their
own series of
wavelengths.
KIRCHOFF’S RULES - #1

A hot, opaque solid, liquid or highly compressed
gas emits a continuous spectrum. It has no lines
in it.
Kirchoff’s Rules - # 2


A hot transparent gas emits light whose
spectrum shows bright lines. These lines are
called emission lines.
Each element has a unique arrangement of
bright lines
Kirchoff’s Rules - # 3

If the light from a luminous source passes
through a cooler gas, the gas may extract
certain specific energies from the continuous
spectrum. We then see dark lines where the
energy has been removed. These dark lines
are called absorption lines.
THE SPECTRUM OF HYDROGEN GAS
ABSORPTION SPECTRUM
Signature wavelengths appear as dark
lines on an otherwise continuous
rainbow.
Lines appear as dips in the intensity
versus wavelength graph.
EMISSION SPECTRUM
Signature wavelengths appear as bright
lines on an otherwise black background.
Lines appear as peaks in the intensity
versus wavelength graph.
The combination of lines
from a stellar spectrum
allow us to determine
which chemicals are
present and in what
quantities.
For example, by matching the spectrum of iron to the absorption lines
from the Sun, we see that there is iron present in the Sun’s atmosphere.
A grating spectrograph
separates light from a
telescope into different
colors by passing it
through a grating of tiny
parallel grooves.
Peacock feathers are natural gratings.
The electrons in an atom can only
exist in certain allowed orbits with
specific energies. The lines seen
from the chemicals are made when
an electron moves from one energy
level to another.
When an electron moves from a lower
energy level to a higher one, a photon
is absorbed. When an electron
moves from a higher energy level to a
lower energy one, a photon is emitted.
The energy of the photon, and thus its
wavelength, are determined by the
energy difference between the two
energy levels.
Emitted photons sent out in all directions will cause the gas
surrounding a star to glow different colors, depending on
which gases are abundant.
HYDROGEN RICH
CLOUDS GLOW RED.
OXYGEN RICH CLOUDS
GLOW GREEN.
Radial Velocity
The proper motion of a star is its
motion perpendicular to our line of sight
across the celestial sphere. This is so
small that it can only be measured for
the closest stars.
The radial velocity of a star is
its motion along our line of
sight either toward or away
from us. Using the spectrum,
we can measure this for
nearly every object in space.
Recall that the wavelength of
light, and therefore the
wavelength of the photons that
light contains, is slightly shifted
when the source is traveling
toward or away from the
observer—the Doppler Effect.
Stars moving toward us show spectral lines that are shifted to blue.
Stars moving away from us show spectral lines that are shifted to red.
The amount of the shift increases with the radial speed.
The Balmer series lines from the spectrum of the star Vega are all shifted toward the
blue side by the same amount. From this we determine that Vega is moving toward us
(blueshift) with a speed of 14km/s (determined from the amount of the shift).
We can examine the proper
motion of nearby stars over long
periods of time.
This picture is made from three
overlapping photographs taken
over a four-year period.
The three dots in a row are
Barnard’s Star seen moving
over the four-year period.