6.1.1 Photons, Photoelectric Effect, and Particle Nature of Light Did the Greeks Invent...
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Transcript of 6.1.1 Photons, Photoelectric Effect, and Particle Nature of Light Did the Greeks Invent...
6.1.1 Photons, Photoelectric Effect, and Particle Nature of Light
Did the Greeks Invent Everything?!?
The Birth of Classical Physics• The ancient Greeks were certainly not the first to
wonder about and investigate nature, but they were the first to leave written records of their ideas.
• They recorded ideas regarding a vast number of subjects from Astronomy to Zoology.
• They conceptualized the building blocks of matter – which they called the ‘atom’ -- literally means ‘cannot be cut’.
585 BCE – 1900 CE• Beginning with the Greeks and
continuing until around 1900, the system of “Classical Physics” slowly emerged with these fundamental ideas at its core…
– Newton’s Laws define mechanics– Light is a WAVE– Atoms make up matter and are
indivisible
• In around 1900, physicists were announcing that physics is a COMPLETE SCIENCE and can explain EVERYTHING!
Well… almost everything…• Physicists in this era were still unable to
explain THREE important phenomenon
Blackbody RadiationWave Theory: A blackbody will emit
radiation that directly relates to its temperature. The higher the
temperature, the more frequencies can be emitted.
Experiment: The intensity of radiation does not keep increasing but forms a
curve of intensities below the expected amount.
Well… almost everything…• Physicists in this era were still unable to
explain THREE important phenomenon
Bright Line SpectrumWave Theory: When you heat an
element it should radiate a continuous range of colors related to that temperature (spectrum).
Experiment: Each element emits only very specific colors in a ‘bright line
spectrum’
Well… almost everything…• Physicists in this era were still unable to
explain THREE important phenomenon
Photoelectric EffectWave Theory: Increasing intensity should
give the same number of faster electrons. Increasing frequency should
give more electrons per second.
Experiment: Increasing intensity gives more electrons per second. Increasing
frequency gives faster electrons or makes the effect stop working.
Planck’s Constant• Max Planck invented the concept of the
QUANTUM to explain the problem of Blackbody Radiation.
• Derived the constant 6.63 x 10-34 J·s to describe the scale of energy increments allowed in atoms.
• Energy going into and out of atoms is QUANTIZED. By why?
Photoelectric Effect• Albert Einstein uses the concept of the
quantum to explain the Photoelectric Effect.
• Invents the idea of the PHOTON which has an energy related to Planck’s Constant and its frequency.
hc
hfEphoton
Equation
Einstein’s Model
• For Einstein’s explanation to be correct light is NOT a wave! It must be made of particles. The particles carry a certain amount of energy related to the light’s frequency.
metal plate
beam of light
a brighter beam would mean MORE
photons
Each photon has a chance to liberate an electron
but the photon must have enough energy (the right frequency)
So, we have phenomenon that show that light MUST be a wave…
ex. DIFFRACTION and INTERFERENCE
…but the PHOTOELECTRIC EFFECT shows that light MUST be made up of particles…
SO WHICH IS IT?!?
EM Radiation (light) is neither waves nor particles but has properties of BOTH.
Photon Properties• Massless• Carry energy and momentum• Travel at the speed of light• Have particle-like collisions – COMPTON EFFCT
e-
e-
v
v = 0
Before After
incoming photon
electron electrongains KE
outgoing photonlower E, lower f
longer λ
Example #1• What is the energy of a photon with a
wavelength of 4.5 x 10-7 meters? What type of photon is it?
Ephoton = hf = hc/λ
Ephoton = (6.63 x 10-34 J·s)(3 x 108 m/s) / (4.5 x 10-7 m)
Ephoton = 4.4 x 10-19 J
Visible Light
Example #2• If a photon has 7.0 x 10-3 joules of energy, what
frequency does it have? What type of photon is it?
Ephoton = hf = hc/λ
7.0 x 103 J = (6.63 x 10-34 J·s) f
f = 1.1 x 1031 Hz
Gamma Ray
Wave/Particle Duality• 1924 Louie de Broglie: because light has
properties of particles, it might be possible for matter to exhibit the properties of waves.
• This was proven to be true by ELECTRON DIFFRACTION experiments demonstrated by Davisson and Germer in 1927.
• While particles do seem to have a particle nature, this is only evident for FAST-MOVING, SUB-ATOMIC PARTICLES.
Mass/Energy Duality• In 1905, Einstein publishes his paper on Special
Relativity.• One of several proposals from this paper is that:
MASS ENERGY AND ENERGY MASS.
2mcE
Equation