Electromagnetic Radiation

Definitions

Electromagnetic Radiation is energy with wavelike characteristicsMoves at a speed of 3.0 x 108 m/s

Wavelength (λ) is the distance between identical points on successive waves – metersFrequency (ν) is the waves passing per second – unit is the Hertz (Hz)

Each type of radiant energy has its own characteristic frequency and wavelengthShort wavelength then high frequencyLong wavelength then low frequency

EM Equation

c = νλ

Yellow light given off by a sodium lamp has a wavelength of 589 nm. What is its frequency?

Radiation of high frequency has more energy than that with low frequency

Quantum Theory

Rules that govern the gain and loss of energy from an object

Max Planck

Said energy comes in “chunks” called quantumEnergy of the quantum depends on the frequency ΔE = hν Planck’s constant (h) is 6.63x10-34 J-s

Energy Lost or Gained

Total energy lost or gained is a multiple of the quantum

ΔEtot = nhν n – number of quantum

Example 1

Calculate the smallest increment of energy that an object can absorb from yellow light with a wavelength of 589 nm.

Example 2

A laser emits light pulses of frequency of 4.69x1014 Hz and deposits 1.3x10-2 J on energy each pulse. How many quantum of energy is this?

Photoelectric Effect

Light shining on certain metals emit electrons A minimum frequency of light is necessaryAlbert Einstein explained this effect with quantum theory

Quantum of light is called a photon Ephoton = hν

When photons are absorbed by a metal, energy is transferred to the electronsIf sufficient energy, the electron can overcome the attractive forces holding it in the atom and escape

Amount of energy depends on the freq.If freq is too low, not enough energy for electron to escapeIf freq is higher than what is needed for electron to escape, the extra energy is converted into kinetic energy making the electron move faster

Total energy of photon = energy required to free electron + kinetic energy hν = EB + EK

EB : called “binding energy” or “work function”

Example

Potassium metal must absorb a wavelength of 540 nm (green) or shorter in order to emit an electron. What will be the kinetic energy of an electron if it absorbs 380 nm (UV) light?