Transcript Modern Physics

Modern Physics
•Previously we showed that Light behaves like
sound. It has characteristics of waves
•Now we get to see how it also behaves like a
particle
•Think about what happens when we turn up
the volume of our MP3 players in terms of
energy
How do we define light
Phenomenon
Waves
Particle
Reflection
Yes
Yes
Refraction
Yes
Yes
Interference
Yes
No
Diffraction
Yes
No
Polarization
Yes
No
Photoelectric Effect
Yes
The Photoelectric Effect
• If we shine light onto the surface on a metal
electron are ejected.
• http://phet.colorado.edu/web-pages/simulationsbase.html
Wave - Particle Duality
• So what are we shooting at the metal to eject
electrons?
• What particles make up light?
– Photons
– The basic unit of electromagnetic energy is known as a
photon. A photon is a massless particle of light that
carries both energy and momentum.
– http://www.physchem.co.za/Light/Particles.htm#Exp
eriment%201
– What is an eV ? The amount of energy required to
move an electron through 1 volt
How much energy is in a Photon?
Sample Problem:
• What is the energy, in Joules of a photon whose energy is
2.11 electronvolts?
– Answer: 1 eV = 1.60x10-19 J thus 2.11 x 1.60x10-19 =
• 3.38x10-19 J
• What are the units?
– What variable does this represent
– What kind?
Light and Energy
• Using the Energy (E) calculated form the previous page,
can we now calculate the frequency of the photon in the
example? Yes!
E = hf so…
3.38x10-19 J = 6.63x10-34 Js x f
f = 5.10 x 10 14 Hz
What color is this?
Determining the work function
Metal 2
• If we plot Frequency vs K.E. we get a
specific graph for each metal
Threshold Frequency
• Will all metals give off electrons with the same
frequency of light?
– No way, nada, nunca
• Each metal has a specific threshold frequency fo, if
graphed, the slope will represent planks constant
(h), the x intercept is fo
– Increasing the intensity will not effect the number of e
emmitted
• Note: this contradicts the wave theory of light
Energy of a Photon
• The energy for each individual photon can be calculated
using…
E photon= hf or = hc/λ
The equation states that the energy of a photon is directly
proportional to the frequency and inversely proportional
to the wavelength
• So what is h?
– Planks constant guess where it can be found
Momentum of a photon
This is very important, so listen! When a photon of visible
light, strikes a metal surface, the photon’s energy is
completely absorbed and transferred to the emitted
electron. When an x-ray photon and an electron collide,
some of the energy of the photon is transferred To the
electron, and the photon recoils with less energy, less
energy means means that the photon now has a lower
frequency.What does this all mean?
The collision causes a conservation of energy
The photon loses energy and momentum while the electron
gains energy and momentum. (compton effect)
Ephoton = Einitial -Efinal
Using the formula above, it can be determined if energy was
absorbed or given off by an electron or photon. If the calculation
above is a positive number, then the atom emitted a photon, if the
calculation yields a negative number, the atom absorbed a proton.
Example:
What is the energy given off by a hydrogen atom if the
electron jumps form the 6th energy level back to the 2nd
energy level?
Solution:
Ep = -.38 - (-3.40) = + 3.32eV
What color light would this emit with this jump?
Mysterious light
• When we view white light through a diffraction grating what do we
see?
– A rainbow of course
– Specifically, this is the color spectrum (continous)
• Why then do we only see lines when viewing gasses?
•Niels Bohr studied this an concluded:
•an atom can only absorb certain
energies (colors) of light (the
absorption spectrum) and once
excited can only release certain
energies (the emission spectrum)
The Bohr Model
• Bohr used these observations to argue that the
energy of a bound electron is limited certain to
quantities of energy.
• This was given the term "quantized."
• Since only certain energy levels are allowed it is
actually possible to diagram the atom in terms of
its energy levels
The energy states of the electron depend upon its particular orbit.
When an electron is in a particular level, it is in a stationary state..
Each stationary state represents a particular amount of energy and
is known as the energy level.
Excited state
Ground state
The n1 state is the ground state, all of the energy levels above this are
known as excited states. Atoms rapidly lose the energy of their various
excited states and return to the ground state. The lose of photons of
specific frequencies causes spectrum lines characteristic to each element.
Using energy level diagrams
• What is the
wavelength of
photons of light
given out by the
transition from –
1.51 eV to the
ground state (-13.6
eV)?
What wavelength of light is
emitted?
• Energy given out =
• -1.51 eV – (-13.6 eV) = 12.09 eV
• Energy in joules =
• 12.09 eV  1.6  10-19 J/eV = 1.93  10-18 J
• Use l = hc = 6.63  10-34 Js  3.0  108 m/s
E
1.93 x 10 -18 J
l =1.03  10-7 m
•
If a deuterium nucleus has a mass of 1.53
x 10-3 universal mass units less then its
components, this mass represents an
energy of ?
1.
2.
3.
4.
1.38 Mev
1.42 Mev
1.53 Mev
3.16 Mev
More
• Determine the frequency of a photon whose
energy is 3.00  10–19 joule
• If a proton were to combine with an
antiproton, they would annihilate each other
and become energy. Calculate the amount of
energy that would be released by this
annihilation.
A hydrogen atom with an electron initially in the
n = 2 level is excited further until the electron is
in the n = 4 level. This energy level change
occurs because the atom has
(1) absorbed a 0.85-eV photon
(2) emitted a 0.85-eV photon
(3) absorbed a 2.55-eV photon
(4) emitted a 2.55-eV photon
Chapter 27 Summary
• Major Equations: c = F * lambda
–
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–
–
E = hf (h = planks constant)
P = h / lambda
E = KEmax + W0
Lambda = h / c * lambda (de Broglie)
Stopping Potential e V0 = KE max
Major Concepts
• Photons
–
–
–
–
Can knock an e out of an atom
Can collide with an electron and lose energy
Can knock an electron to a higher energy level
Can vanish and produce matter/anti matter pair
Problems of interest
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Photoelectric problems
De Broglie wavelength for matter
Compton Scattering
Atomic energy level problems
History of the atom
• Thompson (1897) plum pudding
• Millikan (1913) e = 1.6 x 10-19 J
• Einstein (1905) E = hf = Ke max Epdepends
on Freq
• Compton (1923) photons have momentum
• deBroglie (1923) Particles act like matter
• Rutherford (1911) nucleus of atom
• Bohr (1913) planetary model energy levels
• Heisenburg (1925) delta x delta p >= 2pie /h
Last Topic
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Nuclear Physics chapter 30 in text
Please read and pay attention to decay series
Major concept is E = mc2
Universal mass unit
Fission vs. Fusion