ch16 electromagnetic radiationparticle.korea.ac.kr/class/2006/phys183-01/ch16_med.pdf · 2006. 5....

Eunil Won Dept. of Physics Korea Univ

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Ch16 Electromagnetic Radiation

Eunil Won Dept. of Physics Korea Univ

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InterferenceInterference pattern occur when waves combine

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DiffractionDiffraction is the tendency of waves to bend around objects or spread out after going through an opening

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Light as electromagnetic wave

Present understanding of all electromagnetic waves : very wide spectrum (Maxwell’s Rainbow)

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Light as electromagnetic wave

Radio wave is transparent in the air transmission is possible

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Example 16.1

A wavelength of 300 m corresponds to EM waves used for AM radio transmission. Calculate the frequency in kilohertz of AM radio wave.

kHz 1000Hz101.0300m

m/s103.0

6

8

=×=

×=

=λcf

Eunil Won Dept. of Physics Korea Univ

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The Photon, the Quantum of Light

In 1905, Einstein proposed: electromagnetic radiation is quantized and exists in elementary amounts (quanta) called photons

The quantum of a light wave of frequency f has energy: E = hf(energy of single photon)

h: Planck constant: h = 6.63 x 10-34 J s = 4.14 x 10-15 eV s

ex) A lamp with 100 W power (wavelength=590 nm). How many photons are emitted per second?

# of photons per second = power / hf = power x c / h x wavelength

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Photoelectric EffectIf a beam of light is directed onto a clean metal surface, the light cause electrons to leave that surface difficult to explain if light has wave nature…

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Photoelectric Effect

sJ1063.6 34 ⋅×=⇒= −hhfE

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Photoelectric EffectMore serious experimental setup:

First photoelectric experiment

1) incident light causes current

2) apply potential difference V : collector C is slightly negatively charged

3) At certain V, there will be no currentV = Vstop (stopping potential)

Kmax : the kinetic energy of most energetic electrons

Kmax = eVstop

Kmax does not depend on the intensity of the

light source (inconsistent with wave nature)

Eunil Won Dept. of Physics Korea Univ

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Photoelectric Effect

Photoelectric effect does not occur below a certain cutoff frequency f0(cannot explain it with wave nature)

2nd Photoelectric Experiment: now we vary the frequency of the incident light and measure Vstop

(cutoff wavelength)

To just escape from the target, e-must pick up a certain energy (properties of the target material: work function)

Einstein summed up the photoelectric experiments as:

(photoelectric equation)

explains the above plot

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Compton Scattering

Part of the photon momentum is delivered to the electron?

Scattered x rays show the change in wavelength (difficult to explain if x rays have wave nature)

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In 1916, Einstein extended his concept of light quanta: a quantum of light has linear momentum

(photon momentum)

Compton Scattering

Scattered x rays showed a shift in wavelength (Compton shift): a fraction of momentum is transfered

Eunil Won Dept. of Physics Korea Univ

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Light as Probability WaveLight can be a wave in classical physicsA fundamental

mystery: It is emitted and and absorbed as photons (in quantum physics)

How can particles make interference patterns?

Single-photon version

: A single-photon version of double-slit experiment (one photon at a time) -> Astonishingly interference fringes still build up,supporting the probability wave nature

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Matter WaveMatter can behave as wave?

In 1924, Louis de Broglie suggested matter waves( A moving matter has wavelength)

ex) K=120 eV electron

ex) Me running v=1m/sX-ray and electron diffraction

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Example 17.2

m101.85m/s)(5.0kg)(7.

sJ106.63:ball Bowling 3534

−−

×=⋅×==mvhλ

m101.2m/s)10kg)(5.010(9.11

sJ106.63 :Electron 10631

34−

−

−

×=××⋅×==

mvhλ

Eunil Won Dept. of Physics Korea Univ

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Heisenberg’s Uncertainty PrincipleThe position and the momentum of a particle cannot be measured simultaneously with unlimited precision

Do not think that the particle really has a sharply defined position: I’m sure you are confused by now :-)

Eunil Won Dept. of Physics Korea Univ

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Barrier Tunnelingelectron with energy E moving toward to a potential barrier (U0) when E<U0

classical physics: the electron is bounced off all the time

quantum physics: in some cases the electron penetrates the barrier

Transmission coefficient : the probability of tunneling of the electron(If T=0.020, 20 out of 1000 electrons will tunnel through)

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Crystalline quartz changes its dimension when an electric potential is applied (piezoelectricity): tip can be moved precisely

Electrons from the sample can tunnel through to the tip : tunnel current can be measured and used as a microscope (STM)

The Scanning Tunneling Microscope (STM)