Light III
The Atom & Spectra
February 12, 2012
Average: 65
Test 1
20
15
10
5
0
0-50
50-60
60-70
70-80
80-90
90-100
Takeaway Message:
•
•
YOU NEED TO STUDY MORE
•
you need to come to class EVERY DAY (TPS questions
will NOT be posted on the website)
•
almost NOBODY is coming to office hours, if you do not
understand something, come ask us
•
several of the questions were same/similar to those in
your lecture tutorials and think-pair-share questions, make
sure you understand these!!! go to office hours, look over
questions later, discuss them with classmates outside of
class. In short, you will have to STUDY OUTSIDE OF
CLASS
•
future tests will be just as hard (if not harder)
There will be opportunity for extra credit, but
you will have to be in class to get it
Recap: Blackbody Curve
The peak of the
blackbody curve tells
you temperature
The overall height of
the blackbody curve
tells you Luminosity
Luminosity depends
on Temperature and
Size
So by observing a star’s energy output as a
function of wavelength we can determine its
temperature
What else can we find out about stars by
looking at their light?
composition (i.e. what its made of): how?
5
An atom
consists of a
small, dense
nucleus
(containing
protons and
neutrons)
surrounded by
electrons
Model
Proposed by
Niels Bohr
1913
Bohr model of Atom
Atoms are mostly empty space
A nucleus is
about 10-15 m in
size and the first
electron orbits
out at 10-10 m
from the center
of the atom – The
size of the electron
orbit is 100,000
times greater than
the size of the
nucleus
So if a nucleus the
size of an orange
(10 cm) was
located at the
center of the
football field,
where would the
electron be?
End Zone?
Grandstands?
On Campus?
In Tucson?
If the electron’s
orbit is 100,000
times bigger than
the nucleus then the
electron would be
10,000 m or 6.21
miles away from
the center of the
Football Field!
Still in Tucson, up in
the foothills shopping
at La Encantada!!
Nucleus contains
Protons and
Neutrons
Nucleus
“Ground” state is
lowest energy state
other states are called
“excited” states
Energy levels
become more closely
spaced further from
the nucleus
Nucleus
Electron
Electrons orbit
the nucleus on
energy levels
These energy
levels are fixed
and discrete
Photons (light-waves) are emitted
from an atom when an electron moves
from a higher energy level to a lower
energy level
Emission
Nucleus
Photons (light-waves) can also be
absorbed by an atom when an electron
moves from a lower energy level to a
higher energy level
Absorption
Nucleus
Hydrogen
hc
E = hf =
λ
Each chemical element produces its own
unique set of spectral lines when it is excited
demo
16
We will study three types of spectra!!!
Hot/Dense Energy Source
prism
Continuous Spectrum
Hot low density cloud of Gas
prism
Emission Line Spectrum
prism
Hot/Dense Energy Source
Cooler low density cloud of Gas
Absorption Line Spectrum
The type of spectrum given off depends on
the objects involved
Law #1 – The excited atoms within a hot dense
object give off light of all colors (wavelengths) and
produce a continuous spectrum -- a complete
rainbow of colors (range of wavelengths) without
any spectral lines.
*Note: although I have only drawn visible light, this is
for *all* forms of light
We will study three types of spectra!!!
Hot/Dense Energy Source
prism
Continuous Spectrum
The type of spectrum given off depends on
the objects involved
Law #2 – The excited atoms within a hot, cloud
of gas give off only particular colors
(wavelengths) of light and produce an
emission line spectrum - a series of bright
spectral lines against a dark background.
We will study three types of spectra!!!
Hot low density cloud of Gas
prism
Emission Line Spectrum
The type of spectrum given off
depends on the objects involved
Law #3 – When the light from a hot dense object
passes through a cool cloud of gas, the atoms
within the cloud can absorb particular colors
(wavelengths) of light and produce a absorption
line spectrum - a series of dark spectral lines
among the colors of the rainbow.
We will study three types of spectra!!!
prism
Hot/Dense Energy Source
Cooler low density cloud of Gas
Absorption Line Spectrum
Atoms & Spectra
• Spectra occur because atoms
absorb and emit photons with
only certain wavelengths
• Electrons have discrete
energy states
Atom & Spectra
• This is the spectrum of
the Sun
• Dark features are
absorption lines
• Tell us about
composition of the
Sun
• The Sun has a hot
dense core
surrounded by a lower
density atmosphere
Lecture Tutorial: Light and Atoms-65-69
AND Analyzing Spectra-71-74
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one another.
Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer, ask
another group.
What physical situation makes
this spectrum?
The type of spectrum given off
depends on the objects involved
Law #3 – When the light from a hot dense object
passes through a cool cloud of gas, the atoms
within the cloud can absorb particular colors
(wavelengths) of light and produce a absorption
line spectrum - a series of dark spectral lines
among the colors of the rainbow.
All stars produce
dark line
absorption
spectra
What can we learn by
analyzing starlight?
• A star’s temperature
• A star’s chemical composition
Tutorial (homework): Types of
Spectra – p.63
• Work with a partner!
• Read the instructions and questions carefully.
• Discuss the concepts and your answers with one another.
Take time to understand it now!!!!
• Come to a consensus answer you both agree on.
• If you get stuck or are not sure of your answer, ask
another group.