How atoms
produce
light
http://www.wbateman.demon.co.uk/asa2sums/sum3.1B/topic3.1B.htm
LIGHT
• A generic term for electromagnetic
radiation (like energy from the sun)
• Small packets of energy that travel
as waves…these packets are called
PHOTONS
• Wave properties for a given photon
are all related to each other
Wave properties
• Since light is a form of energy, we know that it
travels as a wave. Waves have a set of properties
that are related to each other.
•
WAVE A
WAVE B
• Wavelength- The distance between two equal
parts of a wave
• Frequency – The # of waves that pass a point
each second
• Speed – How fast a wave travels
(all light travels at the same speed)
Questions:
WAVE A
WAVE B
• Which of the two light samples above has a
longer wavelength?
• If both light samples are travelling by you at the
same speed, which will have more waves pass
by you in one minute?
• From your answer to # 2, which light has the
higher frequency?
• Are wavelength and frequency related directly or
inversely?
Electromagnetic Spectrum
• Which has longer waves, gamma rays or microwaves?
• Which has shorter waves, orange light or green light?
• Which has shorter waves, violet light or ultraviolet light?
• Which has longer waves, red light or infrared light?
• In the spectrum modeled above, which type of radiation do you think
has the highest energy?
• Based on your answers above, are energy and wavelength directly
or inversely proportional?
• Are energy and frequency directly or inversely proportional?
• Which has higher energy, orange light or green light?
• Which has higher energy, red light or infrared light?
• Which has higher energy, violet light or ultraviolet 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”
Pass out Spectroscopes
and colored pecils
• Aim the vertical slit toward the
incandescent light source (the sun)
• 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.
• (given off)
• 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?
• Do the colors that show up always
show up in the same place?
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 the Bohr model 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
(release) a bundle of energy.
• We see that bundle as a photon!
Line Spectrums
• When heated, the electrons in an
atom are excited. Electrons “jump”
to higher energy levels.
• When they go back down they
release energy (photons) as light.
• This can be recorded as a line
spectrum.
Line Spectrums
• Different elements emit
different wavelengths of light
because each element has a
different electron arrangement.
• Each element has a different
pattern of wavelengths and a
different line spectrum.
Photons and Atoms
• Photon is just the name for a small piece
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.
• Techniques like spectroscopy have
allowed us to discover new elements
like Caesium and Rubidium.
• Helium was discovered by looking at
the line spectrum of the sun during an
eclipse!
Conclusion
• Atoms only emit photons of
specific energies
• That is why we can use
atomic emissions spectra to
identify elements in an
unknown sample.