Quiz 21 November 16, 2009
Some distance galaxies are moving away
from us at constant speeds greater than 0.5c.
The speed of light received on Earth from
these galaxies is
(A) slower
(B) faster
(C) the same
*The speed of light in
vacuum has the same
value c in all
directions and in all
inertial reference
frames.
as the speed of light measured in the inertial
reference frames of the galaxies
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Wave-Particle Duality
1. Isaac Newton (1642–1727): Light is a stream of
particles.
2. Christian Huygen (1629–1695): Light is a wave.
3. Thomas Young (1801): experimental proof that
light is a wave by showing that light exhibits
interference phenomena
4. Augustin Fresnel (1819): submitted a paper on the
wave theory of light to explain diffraction to the
French Academy of Sciences. If correct, there
should be a “Fresnel bright spot”. An Academy
test of this showed its existence.
5. James Clerk Maxwell (1860): wave theory of light
6. Albert Einstein (1905): particle nature of light to
explain the photoelectric effect
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Wave-Particle Duality
Isaac Newton
(1642–1727)
Augustin Fresnel
11/17/09(1819)
Christian Huygen
(1629–1695)
Thomas Young
(1801)
James Clerk
Maxwell(1860)
Albert Einstein
(1905)
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Wave-Particle Duality
Light & Matter can exhibit properties of both
waves and particles.
*Existence of photons
*Quant of energy with particulate qualities
Electromagnetic radiation is quantized &
exists in elementary amounts.
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Quantum of Light
*The least energy a light wave of frequency f can
have is hf, the energy of a single photon.
E = hf
h = Planck’s Constant
h= 6.63 x 10-34 Js = 4.14 x 10-15 eV s
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Application to Ozone
Molecular oxygen (O2) is a diatomic molecule. The energy to
dissociate 1 mol of O2 into 2 mol of atomic oxygen is 8.21
x10-19 J/molecule. Assuming the dissociation energy for one
molecule is supplied by a single photon, find the frequency of
the photon.
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Photo-Electric Effect
•Observation:
• Current does not depend on light
intensity
• Depend on the wavelength of light
•Einstein’s hypothesis:
• Photon has energy E=hf=hc/λ where h
is the Planck’s constant (wave-particle
duality)
• To release an electron from a metal
plate E(photon)> threshold energy
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Photon Range Sensitivity of the Eye
Short wavelength limit : ! = 400 nm
hc 1240 eV " nm
E=
=
= 3.10 eV
!
400 nm
Long wavelength limit : ! = 700 nm
hc 1240 eV " nm
E=
=
= 1.77 eV
!
700 nm
# human eye is sensitive to 1.77 < E(nm) < 3.10
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Photoelectric Effect
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Photoelectric Effect
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Work Function
Wc : minimum energy required to remove a
single electron from a piece of metal
hf > Wc electron emitted
hf < Wc electron cannot escape
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Example
A beam of white light
containing
frequencies
between 4.00x1014 Hz to
7.90x1014 Hz is incident on
a sodium surface which has
a work function of 2.28 eV.
What is the range of frequencies in this beam of light
for which electrons are ejected?
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Example
A beam of white light
containing
frequencies
between 4.00x1014 Hz to
7.90x1014 Hz is incident on
a sodium surface which has
a work function of 2.28 eV.
Find the maximum kinetic energy of the photoelectrons
ejected from this surface.
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Photons have Momentum
hf h
p=
=
c !
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Electromagnetic
Spectrum
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