Transcript Chapter 27- Atomic/Quantum Physics

Chapter 27- Atomic/Quantum
Physics
The Sun

http://soho.nascom.nasa.gov/

Why do we see the sun as yellow instead
of green or blue or pink?
Blackbody Spectrum
Photon Theory of Light

Light is transmitted as tiny particles called
photons

The amount of energy in a photon
depends on its frequency
E  hf 
hc

 pc
h= Planck’s Constant= 6.626 x 10-34 Js
f= frequency of light
c= speed of light
λ= wavelength
p= momentum
The Photoelectric Effect

When light shines on a metal surface,
electrons are emitted from the surface
Photocells (p.829)

When the photocell
is in the dark, the
ammeter reads 0 (no
current)

When light with a
high enough
frequency shines on
the current flows in
the circuit
Photocells




KEmax of the emitted electrons
can be found by reversing the
voltage and making the C
electrode negative
The electrons are repelled by
C, but the fastest electrons will
still make it across
There is a minimum voltage,Vo,
called the stopping voltage. No
current will flow if the voltage
is less than the stopping voltage
Kemax = e Vo
Photoelectric Effect

Einstein’s Theory Predicts:
◦ Increasing the intensity of the light does not make the
electrons go faster.
 This is because although more photons are striking the surface,
they have same energy
◦ Increasing the frequency of the light beam increases the
energy of the photons which changes the maximum KE
of the ejected electrons
KEmax  hf  hf o  hf  Wo
Photoelectric Effect

The work function, Wo, is the minimum
amount of energy necessary to get an
electron off the surface of the metal
E  Wo  hf o
 fo
is the “cutoff” frequency. If the light
beam’s frequency is below that, then no
electrons will be emitted
Compton Effect

A.H. Compton scattered xrays
from various materials
◦ Found out that the scattered
light had a lower frequency
than incident light
◦ Since frequency decreases,
wavelength increases

Used conservation of
momentum to determine that
the photon transfers some of
its energy to the electron
de Broglie Wavelength

Light sometimes behaves like a wave and
sometimes like a particle

Louis de Broglie came up with the idea
that particles might also have wave
properties
h
h
 
p mv
De Broglie wavelength of a particle
de Broglie Wavelength

The wavelength of large objects is very
small

For a 0.20 kg ball travelling at 15 m/s
6.6 x1034 Js
34

 2.2 x10 m
0.20kg15m / s 
de Broglie Wavelength

Determine the wavelength of an electron
that has been accelerated through a
potential difference of 100 V
1 2
qV  mv
2
2qV
6 m
v
 5.9 x10
m
s
h
6.26 x10 34 Js


 1.2 x10 10 m
mv (9.1x10 31 kg)(5.9 x106 m )
s
Davisson-Germer Experiment

The spacing of atoms in a crystals is on
the order of 10-10 m, so one could be
used as a diffraction grating

In 1927, Davisson and Germer scattered
electrons from the surface of a metal
crystal. The wavelength they got matched
the predicted de Broglie wavelength
Bohr Model of the Atom

Electrons orbit the nucleus in circular
orbits called stationary states

When an electron jumps from one state
to another, light is either absorbed or
emitted

The energy required to go between states
is a fixed amount
Bohr Model of the atom

If an electron jumps from a higher state
to a lower state, it emits a single photon
of light
Photon Energy  hf  Eu  El  Energy of upper state - Energy of lower state
Energy Level Diagram (p. 847)

n= 1is ground state,
n=2,3,4.. Are excited
states

To completely free an
electron in the ground
state, you’d need to
put in 13.6 eV of
energy (ionization
energy of Hydrogen)
Energy Level Diagram

How much energy
to go from ground
to n=2?
13.6eV  3.4eV  10.2eV

How much energy
to go from n=2 to
n=4?
3.4eV  0.85eV  2.55eV
Energy Level Diagrams

What are the
possible transitions
for an electron in
excited state n=3?
◦ 31
◦ 32
◦ 21
Emission Spectra

A material’s emission spectrum show the
wavelengths of the photons emitted when
electrons jump to lower energy states
Absorption Spectra

The absorption spectrum of a material
shows that gases can absorb light at the
same frequencies at which they emit
Absorption/Emission Spectra