Transcript Electrons & Light

How atoms
produce
light
http://www.wbateman.demon.co.uk/asa2sums/sum3.1B/topic3.1B.htm
What is Light?
• LIGHT is a form of energy
• Light can be considered as a bunch
of individual light “packets” called
PHOTONS
• Each packet has its own set of
properties (wavelength, etc)
• A bunch of these packets traveling
together is like a ray of light.
Spectroscope
• A simple spectroscope has a flat
prism that separates light so we can
see the individual colors.
http://asd-www.larc.nasa.gov/edu_act/simple_spec.html
“White Light”
• Aim the vertical slit toward the
incandescent light source.
• You will see the light through the slit.
• Without moving the spectroscope, drift
your eyes to the right until you see the
numbers on the scale.
• What do you see over the numbers?
Continuous Spectrum= all colors
There are no “blank spots” in the
spectrum!
http://physics.uoregon.edu/~jimbrau/astr122/Notes/Chapter3.html
Why continuous spectrum?
• A solid is heated…all of its
atoms/molecules and their parts
move really fast
• Energy is given off as the atoms
constantly vibrate.
• Photons of all colors can be emitted.
• All colors blend into “white light”
Another type of spectrum
• Aim the vertical slit at the overhead
lights in the room.
• How does this look different from the
incandescent light?
What kind of spectrum?
• Look at the overhead lights again.
Did anything change?
• Are photons of ALL colors being
given off by these lights?
• This is called a brightline spectrum!
Brightline Spectrum
When only certain photons are observed,
it means that only light packets of a
particular type are being emitted!
• Each photon has a specific energy
value.
• So only certain energy exchanges are
happening within the heated substance.
• So there must only be certain ways of
changing the energy in the substance!
How?
• This can be explained by the movement of
electrons!
• We know from middle school that atoms
have “layers” of electrons called energy
levels.
• Each energy level has electrons with a
certain amount of energy in them that
matches the level.
• When the electrons change levels, they
have to gain or lose energy to do so.
• Each time they lose energy, they emit a
bundle of energy.
• We see that bundle as a photon!
Observing Elements
• You will use the spectroscopes to see
what photons are given off by the
elements used in the flame test.
• There will be some “background” light in
your spectra.
• Only focus on the bright lines!
http://njsas.org/projects/atoms/spectral_lines/1/
Atomic Spectra/
Flame Tests
• When we heat up an element, its
electrons gain and lose energy while
it is being heated.
• The photons released with each
energy loss travel together as rays of
light that blend into a specific color.
• This light can be passed through a
prism so you can see the individual
colors of each type of photon.
Sodium
Sodium Emission Lines
400
500
600
Wavelength in nanometers
http://www.800mainstreet.com/spect/emission-flame-exp.html
700
Analyze the spectrum
• How many different types of photons
are in the visible range for a sodium
atom emission?
– Two! Both are yellow, but have
different wavelengths, so are different
yellows.
Copper
http://en.wikipedia.org/wiki/Flame_test
Copper Emission Lines
400
500
600
Wavelength in nanometers
http://webmineral.com/help/FlameTest.shtml
700
Analyze the spectrum
• How many different types of photons
are in the visible range for a copper
atom emission?
– Eight! A purple one, a blue one, and
several wavelengths of greens and
yellows.
• The individual photon colors emitted by
the electrons in any atom form the
“atomic emission spectrum”
• Also called “brightline spectrum”
http://library.thinkquest.org/19662/low/eng/model-bohr.html
Conclusion
• Atoms only emit photons of
specific energies
• WHY??
All about…
LIGHT
LIGHT
• A form of energy!
• Travels in waves
• Wave properties are all related
• All light is part of the electromagnetic
spectrum (like energy from the sun)
Wave properties
• Speed
• Wavelength
• Frequency
• Energy
http://web.chemistry.gatech.edu/~williams/bCour
se_Information/6582/problem_sets/waves/index.
html
Speed
• Light travels at the speed of light (duh!)
• The speed of light = 2.998 x 108 m/s
• The symbol “c” stands for the speed of
light
– c = 2.998 x 108 m/s
• All light waves will have the same speed,
so speed is a constant
Waves
http://rst.gsfc.nasa.gov/Intro/Part2_2.html
Wave Equations
• Without the energy component
c=λ
– c = 2.998 x 108 m/s
– λ (lambda) = wavelength in meters
– (nu) = frequency in 1/s or s-1 or Hz
Wave Equations
• With the energy component
E=h
or
E = hc/λ
• E = energy in Joules
• h =Planck’s constant= 6.636x10-34 J·s
Quantum
• A specific quantity of light
• Bohr said that when energy is added to
atoms, the electrons gain a “quantum” of
energy to move to a higher level.
• When electrons relax back to their normal
state, they emit a quantum of energy to go
back to the lowest level.
Quantum…photon
• Photon is just the name for a quantum of
light
• Electron Transition – when an electron
moves from one level to another
– When an electron transitions to a higher
energy level, a photon is absorbed.
– When an electron transitions to a lower energy
level, a photon is emitted.
Quantum…photon
• The emitted photon is just a “piece”
of light.
• It has a specific energy value, so it
has a specific wavelength, frequency
and color
• If you can measure the wavelength
of the photon, you can calculate its
energy.
Example
• The photon released by a certain electron
transition has an energy of 4.56x10-19 J.
Calculate the wavelength and frequency of this
light. Is it in the visible range?
• E=h
so
=E/h
• (4.56x10-19 J)/(6.626x10-34J·s)
• = 6.88x1014Hz
• c=λ
so λ=c/
• (2.998x108m/s)/(6.88x1014Hz)
• = 4.36x10-7m = yes in the visible range
Gas Discharge Tubes
• Another way to give energy to the
atom is using electricity
http://www.physics.lsa.umich.edu/demolab/graphics2/7b10_10.jpg
Gives a spectrum just like
that of a flame…
Figure 5.12 in your textbook
Colors…energy?
• Once you read the wavelength from
your spectroscope scale, you can
calculate the energy the electrons
had to lose in order to release that
color of photon.
Ta-daa
• This is why scientists can calculate
the energy values of the levels within
an atom even though they can’t see
them!
• Please complete the packet,
“Analysis of Spectral Lines”