Transcript PowerPoint 簡報 - Maastricht University

Young/Freeman
University Physics 11e
Ch 38 Photons,
Electrons, and Atoms
© 2005 Pearson Education
38.1 Emission and Absorption of Light
Line spectra
 Photoelectric Effect
 X-Rays
 Photons and Energy Levels

© 2005 Pearson Education
38.1 Emission and Absorption of Light
Continuous spectrum
© 2005 Pearson Education
Line spectrum
Hydrogen
Carbon
Sodium
38.2 Photoelectric Effect

When light is incident on certain metallic surfaces,
electrons are emitted from the surface




This is called the photoelectric effect
The emitted electrons are called photoelectrons
The effect was first discovered by Hertz
The successful explanation of the effect was given by
Einstein in 1905

Received Nobel Prize in 1921 for paper on electromagnetic
radiation, of which the photoelectric effect was a part
Photoelectric Effect Schematic

When light strikes E,
photoelectrons are
emitted

Electrons collected at C
and passing through the
ammeter are a current in
the circuit

C is maintained at a
positive potential by the
power supply
Photoelectric Current/Voltage
Graph



The current increases with
intensity, but reaches a
saturation level for large
ΔV’s
No current flows for
voltages less than or equal
to –ΔVs, the stopping
potential
The stopping potential is
independent of the
radiation intensity
Features Not Explained by
Classical Physics/Wave Theory




No electrons are emitted if the incident light frequency
is below some cutoff frequency that is characteristic of the
material being illuminated
The maximum kinetic energy of the photoelectrons is
independent of the light intensity
The maximum kinetic energy of the photoelectrons
increases with increasing light frequency
Electrons are emitted from the surface almost
instantaneously, even at low intensities
Einstein’s Explanation





Energy from the light beam is transferred to the electrons in the
solid by photons which have an energy related to the freqency of
the beam.
The photon’s energy would be E = hƒ
Each photon can give all its energy to an electron in the metal
The electron is considered to be in a well of height f which is
called the work function of the metal
Because of energy conservation the maximum kinetic energy of
the liberated photoelectron is
KE = hƒ – Φ
Explanation of Classical
“Problems”

The effect is not observed below a certain cutoff
frequency since the photon energy must be greater than
or equal to the work function
 Without this, electrons are not emitted, regardless of
the intensity of the light

The maximum KE depends only on the frequency and
the work function, not on the intensity

The maximum KE increases with increasing frequency

The effect is instantaneous since there is a one-to-one
interaction between the photon and the electron
38.3 Atomic Line Spectra and Energy Levels
energy of
emitted photon
hc
hf 
 Ei  Ef
λ
© 2005 Pearson Education
The hydrogen spectrum
energy levels of
the hydrogen atom
hcR
En   2
n
© 2005 Pearson Education
n  1, 2, 3, 4, ....
1 1 
 R  2  2  n  2,3, 4,

1 n 
1
 1 1 
 R  2  2  n  3, 4,5,

2 n 
1
Lyman series
Balmer series
Paschen series
Brackett series
Pfund series
1 1 
 R  2  2  n  4,5, 6,

3 n 
1
 1 1 
 R  2  2  n  5, 6, 7,

4 n 
1
1 1 
 R  2  2  n  6, 7,8,

5 n 
1
© 2005 Pearson Education
38.5 The Bohr Model of an atom
Classical physics prediction
Electron should
continuously
radiate
electromagnetic
waves and spiral
into the nucleus
© 2005 Pearson Education
Bohr model
Proton is assumed to
be stationary; the
electron revolves in a
circle of radius rn with
speed vn
© 2005 Pearson Education
In the Bohr model of the hydrogen atom, the
permitted values of angular momentum are integral
multiples of h/2π. The integer multiplier n is called
the principal quantum number for the level. The
orbital radii are proportional to n2 and the orbital
speeds are proportional to 1/n. (See Example 38.6)
© 2005 Pearson Education
Energy level of different atoms
© 2005 Pearson Education
38.6 The Laser
© 2005 Pearson Education
Population inversion
© 2005 Pearson Education
38.7 Production of X-rays

X-rays are produced when
high-speed electrons are
suddenly slowed down



Can be caused by the electron
striking a metal target
A current in the filament
causes electrons to be
emitted
These freed electrons are
accelerated toward a dense
metal target
Production of X-rays


An electron passes near a
target nucleus
The electron is deflected from
its path by its attraction to the
nucleus


This produces an acceleration
It will emit electromagnetic
radiation when it is accelerated
The maximum x-ray energy, and minimum wavelength
results when the electron loses all its energy in a single
collision, such that
eDV = hfmax = hc/min or therefore min
hc

eDV
The laser operates on the principle of stimulated
emission, by which many photons with identical
wavelength and phase are emitted. Laser operation
requires a non-equilibrium condition called a
population inversion, in which more atoms are in a
higher-energy state than are in a lower-energy state.
© 2005 Pearson Education
X-ray
spectrum
38.8 The Compton Effect


Compton directed a beam of x-rays toward a block of
graphite
He found that the scattered x-rays had a slightly longer
wavelength that the incident x-rays




This means they also had less energy
The amount of energy reduction depended on the
angle at which the x-rays were scattered
The change in wavelength is called the Compton shift
The Compton shift depends on the scattering angle and
not on the wavelength
Compton scattering
© 2005 Pearson Education
© 2005 Pearson Education
© 2005 Pearson Education
38.8 Blackbody Radiation

An object at any temperature is known to emit
electromagnetic radiation
Sometimes called thermal radiation
 Stefan’s Law states that the total power radiated is given
as



P = s T4
The wavelength λm at which a blackbody radiates most
strongly is inversely proportional to T. The Planck
radiation law gives the spectral emittance I(λ) (intensity
per wavelength interval in blackbody radiation.
The total radiated intensity (average power radiated
per area) from a blackbody surface is proportional to
the fourth power of the absolute temperature T. The
quantity s  5.67 108W / m2  K 4 is called the StefanBoltzmann constant. The wavelength λm at which a
blackbody radiates most strongly is inversely
proportional to T. The Planck radiation law gives the
spectral emittance I(λ) (intensity per wavelength
interval in blackbody radiation.
© 2005 Pearson Education
END
© 2005 Pearson Education