Transcript L 35 Modern Physics [1]

L 33 -Modern Physics
• “Modern” – 20th Century
• By the end of the 19th century it seemed
that all the laws of physics were known
• However, there were a few problems
where classical physics (pre-20th century)
didn’t seem to work.
• It became obvious that Newton’s laws
could not explain atomic level phenomena
Newton’s Laws have flaws!
• Newton’s laws, which were so successful
in allowing us to understand the behavior
of big objects such as the motions of the
planets, failed when pushed to explain
atomic size phenomena.
• The discovery of the laws of atomic
physics led to every important 20th
century discovery that have transformed
our lives, the electronic revolution.
Newton’s laws also fail at high velocities
• According to Newton’s laws, if
we keep exerting a force on an
object, it will continue to
accelerate and there is no limit
to how fast the particle can go
• According to Einstein, nothing
can be accelerated to a speed
greater than the speed of light,
186,000 miles per second
ATOMS and classical physics
• According to the laws of
mechanics and electricity
and magnetism, an
orbiting electron in an
atom should continually
radiate away energy as
electromagnetic waves.
• Very quickly the electron
would loose all of its
energy and there would
be no atoms!
accelerated charges radiate energy
The photoelectric effect – defies a
classical explanation!
LIGHT
photoelectrons
Metal plate
• When light shines on a metal surface, electrons
pop out – we call these photoelectrons
• Photoelectrons are only emitted if short
wavelength light (blue) is shined on the metal, no
matter how intense the light is.
Details of a photocell
Photocells used as a safety device
The child interrupts the
beam stopping the current
Einstein explains the PE effect,
receives Nobel Prize in 1921
• Light is an electromagnetic wave, but when
it interacts the metal it behaves like a
particle, a light particle called a photon.
• A beam of light is a beam of photons.
number
Photon Energy =
Wavelength
• Blue photons are more energetic than red
photons, so blue photons can knock
electrons out of the metal
The quantum concept
• The photon concept is a radical departure
from classical thinking.
• In classical physics, energy can come in
any amounts
• In modern physics, energy is QUANTIZED
 comes in definite packets  photons
• In the PE effect energy is absorbed by the
electrons only in discreet amounts
Video recorders and
digital cameras
A CCD (charge coupled
device) can be used
to capture photographic
Images using the
photoelectric effect.
http://money.howstuffworks.com/camcorder2.htm
The quantum concept
and the Bohr Atom
• Niels Bohr, a Danish physicist, used the
quantum concept to explain the nature of
the atom.
• Recall that the orbiting electrons,
according to classical ideas, should very
quickly radiate away all of its energy
• If this were so, then we would observe
that atoms emit light over a continuous
range of wavelengths (colors) NOT SO!
The Bohr Atom
Nucleus
+
Ei
Ef
The orbits farther from
the nucleus are higher
energy states than
the closer ones
• The electrons move in
certain allowed, “stationary”
orbits or states in which
then do not radiate.
• The electron in a high
energy state can make a
transition to a lower energy
state by emitting a photon
whose energy was the
difference in energies of the
two states, hf = Ei - Ef
Line spectra of atomic hydrogen
The Bohr model was successful in determining
Where all the spectral lines of H should be.
Line spectra of atoms
Line spectra are like atomic fingerprints. Forensic
scientists use line spectra to identify substances.
THE LASER: a product
of 20th Century Physics
Light
Amplification by
Stimulated
Emission of
Radiation.
Laser Type
Wavelength (nm)
Argon fluoride (UV)
193
Krypton fluoride (UV)
248
Xenon chloride (UV)
308
Nitrogen (UV)
337
Argon (blue)
488
Argon (green)
514
Helium neon (green)
543
Helium neon (red)
633
Rhodamine 6G dye (tunable)
Ruby (CrAlO3) (red)
Nd:Yag (NIR)
Carbon dioxide (FIR)
570-650
694
1064
10600
What’s special about laser light?
Laser Light
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stimulated light emission by atoms
Monochromatic (one pure color)
coherent – all the waves travel in step
all waves travel in same direction
waves reinforce each other
applications
– scanners and CD readers, CD burners
– cut metals
– laser surgery
Medical Applications of Lasers
Laser surgery to correct for
(a) nearsightedness, and
(b) farsightedness
How does a CD burner Work?
• infrared laser light is applied to a layer of
photosensitive dye on top of the plastic
• this causes the dye to darken (no burning!)
• by selectively darkening particular points
along the CD track, and leaving other
areas of dye translucent, a digital pattern
is created that can be read by a standard
CD player
http://computer.howstuffworks.com/cd-burner4.htm
Laser
Fusion
Multiple beams of a powerful laser are focused
on a tiny pellet containing fusion fuel. The
laser energy compresses the pellet producing
a mini-hydrogen bomb that produces energy
Medical Imaging Techniques
• x-rays
• CT and CAT scans (Computerized
Tomography)
• MRI’s (Magnetic Resonance Imaging)
X-rays
x-ray of Homer’s head
• very short wavelength
(0.01 – 0.1 nm)
electromagnetic waves
• produced when
energetic electrons
slam into a metal target
• able to penetrate soft
tissue, but not bone
• produces a two
dimensional shadow
image
A pineapple and a banana
• A shadow image can be
misleading
• two shadows taken from
different angles provides
a better picture
• shadows taken at
multiple angles gives a
more complete picture
• this is what a CT or CAT
scan does
CAT Scans
X ray images are taken at many different angles
passing through the patient. Some of the cuts
overlap. A full three dimensional image can be
reconstructed using computers.
 this procedure is called tomography.
Computerized Tomography
• A computerized tomography
or CT scan image is formed
by analyzing x-ray shadow
images taken at many
different angles and
positions
• an x-ray source and an array
of electronic detectors
rotates around the patient as
the patient slowly moves
through the ring.