The Particlelike Properties of Electromagnetics Radiation Wei-Li Chen 10/27/2014

The Photoelectric Effect The maximum current is proportional to the incident light intensity. There is no minimum intensity below which the current is absent. V 0 is independent of the incident light intensity. Higher frequency will cause higher V s. Current occurs when the light frequency is above certain value. This threshold frequency varies for different target materials. There is no time lag between the turning on of the light and and the appearance of the electron. Stopping potential

The Photoelectric Effect

Einstein’s Photoelectric Effect Equation

X-ray Tube

X-ray Spectra Bremsstrahlung (braking radiation) Characteristic spectra

X-ray X-rays were discovered in 1895 by W. Roentgen. They are electromagnetic radiation whose wavelength is near the range from 1nm to 0.001nm. An X-ray photon carries energy 1keV or more. When high energy electrons hit metal target, the deceleration of electrons will generate electromagnetic radiation that is called bremsstrahlung. The most of the radiation are emitted near 90 degree to the direction of the electrons’ path. Strong characteristic X-rays produced by electron transitions between different energy levels superimpose on the bremsstrahlung spectrum. The characteristic peaks vary with different metal targets. The X-ray spectrum drops to zero at a maximum feruency f max which follows Duane-Hunt law:

Bragg Law The size of the atoms in a crystal (~0.1nm) is compatible to the wavelength of X-ray. In 1912 Laue suggested that a crystal can be used as 3D grating for X-ray. Bragg diffraction law: Historically Bragg diffraction is an important tool to study X-rays. Today Bragg diffraction is used to characterize the crystal structure and lattice constant.

Bragg Law Crystalline planes

X-ray Diffraction Taylor fig.4.10

Supplement for X-ray Diffraction

The Compoton Effect Electromagnetic radiation interact with an electron will cause it to oscillate at the same frequency. Therefore, according to classical physics, the wavelength of the electromagnetic radiation after interact should remain constant. However, the scattered radiation wavelength becomes longer after collision. Compton derived the wavelength change of a photon after its collision with an electron based on energy and momentum conservation laws. The Compton effect proves that photons possess momentum, which is an essential characteristic of particles.

θ θ Serves as the detector

Compton wavelength

Experiment setup

Optical Spectrum Analyzer Dispersion n(λ) n 1 sinθ 1 =n 2 sinθ 2 red blue

Electromagnetic Wave Spectrum

Blackbody Radiation The technology that utilize the dispersion of light into different colors (wavelength) is called spectroscopy. When an object reaches thermal equilibrium with the environment, its temperature will be constant. It continuously absorb and emit radiation with equal amount. The electromagnetic radiation emitted by such an object is called thermal radiation. A body that absorbs all radiation incident on it is called an ideal blackbody. - Stefan-Boltzmann law: The power per unit area radiated by an blackbody. - Wien displacement law: λ m is wavelength of maximum emission. Any body that emits radiation at each wavelength in a constant ratio less than unity to that emitted by a black body at the same temperature. An opaque object under equilibrium will emit less radiation than a blackbody. The ratio is called emissivity ε.

u(λ)Comments Plank’s Law 8πhcλ -5 / [exp(hc/kλT) – 1] match with experimental data Raleigh-Jeans Law 8πkTλ -4 good approximation at long wavelength only  ultraviolet catastrophe Wien Law c 1 λ -5 exp(-c 2 /λT) good approximation at short wavelength only Blackbody Radiation Formula

ultraviolet catastrophe works only for long wavelengths Wien law works only for short wavelengths (Raleigh-Jeans law)

The difference between both theories

Density of Oscillation Modes

Other Photon Processes

Pair Production Annihilation

What is a photon Like EM wave, velocity is c. No mass and rest energy Carrying energy and momentum Can be created and destroyed. Can collide with other particles.

When a photon detected by the detector, it shows particle-like property. When it passes through the switch, it shows wave-like property. Which Slit Does the Electron Go Through?