Eunil Won Dept. of Physics Korea Univ

1

Ch16 Electromagnetic Radiation

Eunil Won Dept. of Physics Korea Univ

2

InterferenceInterference pattern occur when waves combine

Eunil Won Dept. of Physics Korea Univ

3

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

Eunil Won Dept. of Physics Korea Univ

4

Light as electromagnetic wave

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

Eunil Won Dept. of Physics Korea Univ

5

Light as electromagnetic wave

Radio wave is transparent in the air transmission is possible

Eunil Won Dept. of Physics Korea Univ

6

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

7

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

Eunil Won Dept. of Physics Korea Univ

8

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…

Eunil Won Dept. of Physics Korea Univ

9

Photoelectric Effect

sJ1063.6 34 ⋅×=⇒= −hhfE

Eunil Won Dept. of Physics Korea Univ

10

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

11

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

Eunil Won Dept. of Physics Korea Univ

12

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)

Eunil Won Dept. of Physics Korea Univ

13

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

14

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

Eunil Won Dept. of Physics Korea Univ

15

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

Eunil Won Dept. of Physics Korea Univ

16

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

17

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

18

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)

Eunil Won Dept. of Physics Korea Univ

19

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)