Photon phys org Electromagnetic Waves Electromagnetic waves are

Photon phys. org

Electromagnetic Waves ] Electromagnetic waves are characterized by their wavelength or frequency. • Linked by the speed of light ] Accelerating charges emit electromagnetic waves. • Based on frequency of oscillation Emitted light accelerating charge

Electromagnetic Spectrum ] The electromagnetic spectrum is divided into bands • Typical upper limit listed • e. V is the amount of energy to raise 1 electron by 1 volt Radio waves Microwaves Infrared Visible light Ultraviolet light X-rays Gamma rays l (m) 1 1 10 -3 0. 75 10 -6 0. 4 10 -6 1. 2 10 -8 1. 4 10 -11 f (Hz) E (e. V) 3 108 1. 24 10 -6 3 1011 1. 24 10 -3 4 1014 1. 65 7. 5 1014 3. 1 2. 4 1016 1 102 3 1019 1. 2 105 (highest energy)

Accelerating Charge ] An AC power source creates an oscillating current. ] The charges in the conductor accelerate. ] This produces a changing electric field. • Creates changing magnetic field • Produces EM wave. ] This is an antenna.

Radio Waves ] Radio waves come from electrical oscillations. • Microwaves with shorter wavelengths ] The long wavelengths travel easily and are not interfered by atoms. l n Radio waves > 1 m < 3 x 108 Hz Microwaves 1 mm to 1 m 3 x 1011 Hz to 3 x 108 Hz

Radiation from Heat ] Heated objects give off electromagnetic waves. • Blackbody radiation • Higher temperature has more radiation ] intensity low energy high energy A hot object gives off a spectrum of frequencies. frequency

Infrared and Ultraviolet ] Visible light is often produced by vibrating atoms and molecules. • Infrared lower energy than visible light • Ultraviolet higher energy than visible light l Infrared Visible light Ultraviolet n 700 nm to 1 mm 400 nm to 700 nm 10 nm to 400 nm 4 x 1014 Hz to 3 x 1011 Hz 8 x 1014 Hz to 4 x 1014 Hz 3 x 1016 Hz to 8 x 1014 Hz

High Energy Photons ] X-rays come from energetic transitions in atoms. ] Gamma rays come from nuclear and subatomic reactions. Thinkstock. com l n X-rays 0. 01 to 10 nm 3 x 1019 Hz to 3 x 1016 Hz Gamma rays < 0. 01 nm > 3 x 1019 Hz

Kirchhoff’s Radiation ] Radiated electromagnetic energy is the source of radiated thermal energy. • Depends on wavelength ] Objects can emit and absorb electromagnetic energy. • Emission coefficient el • Absorption coefficient al ] The distribution Il depends on temperature.

Blackbody ] A black object is perfectly absorbing. • Absorption coefficient is 1 ] The distribution is just due to emission. ] An isolated cavity with a narrow hole radiates like a perfectly black body at the same temperature (1859).

Ultraviolet Catastrophe ] Classical thermodynamics predicted that the power emitted would increase at higher frequency (1884). intensity low energy high energy • Infinite energy might be emitted ] Real data did not match this conclusion. frequency

Planck’s Law ] ] Planck assumed that oscillating charges emit only discrete energies (1900). ] Planck established a relationship between the energy and frequency. • Planck’s constant h Energy is quantized. Emitted photon ] accelerating charge Planck solved the Kirchhoff problem and UV catastrophe.

Ejected Electrons ] Electromagnetic energy could eject electrons from the surface. I • Accelerate with voltage • Measure current ] Classical expectation is that the current varies with intensity. • May also vary with frequency. e V

Stopping Potential ] As the voltage is decreased the photocurrent decreases. I • Matches classical view ] The voltage can be reversed and some current flows. • Stopping point independent of intensity • Characteristic of metal e V

Photoelectric Effect ] g • Photon absorbed • Electron released e Z A photon can eject an electron from an atom. ] Energy within the atom Z is quantized. • Minimum energy f needed for interaction ] Einstein’s photoelectric equation requires light to be a particle!

Photocell ] The photoelectric effect is commonly used to measure light. • Camera light meter ] It can also generate electricity. • Photovoltaic cell

Cathode Rays ] Electrons can be accelerated toward a metal target. • Heated cathode to create electrons • High voltage to get high energy electrons ] Beam deflection by a magnet led to the discovery of the electron as a particle. I e V

X-Rays ] The photons come of with a spectrum of energy. • Called x-rays • Maximum corresponds to potential ] Peaks correspond to specific quantized energies in an atom.

Scattering ] High energy photons can scatter off atoms in the Compton effect. • Photon changes energy • Electron ejected g g ’ e ] High energy electrons can scatter off atoms to create bremsstrahlung. • Gamma ray photon g q f e e Z