Download L 35 Modern Physics [1]

L 35 Modern Physics [1]
• Introduction- quantum physics
• Particles of light  PHOTONS
• The photoelectric effect
– Photocells & intrusion detection devices
• The Bohr atom
– emission & absorption of radiation
– LASERS
Sometimes light behaves like a particle and
sometimes particles behave like waves!
Modern Physics- Introduction
• “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
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
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
Electron Kinetic Energy
19
10
2
v (classical)
18
2
10
v (relativistic)
2
v (experiment)
17
c2
10
16
10
15
10
14
10
-16
10
KE (classical) = ½ mv2
A bad haircut can make
anybody-14look stupid.
-15
-13
-12
10
10
10
Kinetic Energy (J)
10
The photoelectric effectphotons
LIGHT
photoelectrons
Metal plate
• When light shines on a metal surface, electrons
pop out
• Photoelectrons are only emitted if the wavelength
of the light is shorter than some maximum value,
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
Photoelectric effect defies a
classical explanation
• According to classical physics, if the
intensity of the light is strong enough,
enough energy should be absorbed by the
electrons to make them pop out
• The wavelength of the light should not
make a difference.
• What is going on ? ? ?
Einstein explains the PE effect,
receives Nobel Prize in 1921
• A radical idea was needed to explain the
photoelectric effect.
• Light is an electromagnetic wave, but
when it interacts with matter (the metal
surface) it behaves like a particle, a light
particle called a photon.
• A beam of light is thought of as a beam
of photons.
Photoelectric effect – PHOTONS
• The energy of a photon depends on the
wavelength or frequency of the light
• Recall that speed of light
= wavelength () x frequency (f)
• Photon energy
E = Planck’s constant x frequency = h f
• Shorter wavelength (higher f) photons
have a higher energy
PE – explanation continued
• A certain amount of energy is required to
make an electron pop out of a metal
• An photoelectron is emitted if it absorbs a
photon from the light beam that has
enough energy (high enough frequency)
• No matter how many photons hit the
electron, if they don’t have the right
frequency the electron doesn’t get out
Blue and red photons - example
• How much energy does a photon of
wavelength = 350 nm (nanometers) have
compared to a photon of wavelength =
700 nm?
• Solution: The shorter wavelength photon
has the higher frequency. The 350 nm
photon has twice the frequency as the
700 nm photon. Therefore, the 350 nm
photon has twice the energy as the 700
nm photon.
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 of
energy h f.
• 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 LASER: a product
of 20th Century Physics
Light
Amplification by
Stimulated
Emission of
Radiation.
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!
Niels Bohr and his five sons
Line spectra of atoms
Line spectra are like atomic fingerprints. Forensic
scientists use line spectra to identify substances.
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.
Emission and Absorption
• When an electron jumps from a high
energy state to a low energy state it emits
a photon  emission spectrum
• An electron in a low energy state can
absorb a photon and move up to a high
energy state  absorption spectrum
Emission
+
transition to a
lower energy state
Absorption
+
transition to a
higher energy state
Black lights and fluorescence
• some materials can absorb light at one
wavelength (color) and re-emit it at
another wavelength
• a black light emits in the ultraviolet
• fluorescent materials absorb UV and reemit in the visible
visible
UV
black
light
fluorescent material