Light Emission www.physics.sfasu.edu/friedfeld/ch29lec.ppt
Radio waves are produced by electrons moving up and down an antenna. Visible light is produced by electrons changing energy states in an atom.
EXCITATION
Excitation - occurs when an electron in an atom is given energy causing it to jump to a higher orbit. This can happen through Collisions or Photon absorption (the photon absorption must exactly match the energy jump).
Here comes a nucleus With possible orbits for electrons e - In this configuration the atom is not excited.
Here comes a photon e - In this configuration the atom is not excited.
Now the atom is said to be excited because the electron is in a higher than normal orbit. e -
Now the atom is in a non-excited state again. It emitted a photon when it changed orbits. e -
The excited atom usually de-excites in about 100 millionth of a second. The subsequent emitted radiation has an energy that matches that of the orbital change in the atom.
Atomic Excitation
This emitted radiation gives the characteristic colors of the element involved. The atoms do not wear out. Flame Colors
Emission Spectra Continuous Emission Spectrum Slit White Light Source Prism Photographic Film
Emission Spectrum of Hydrogen Slit Discrete Emission Spectrum Film Low Density Glowing Hydrogen Gas Prism Photographic Film
Spectra of the Noble Gases
INCANDESCENCE Blue hot is hotter than white hot which is hotter than red hot. White light - all colors in the visible are present. Electron transitions occur not only in the parent atom but in adjacent atoms as well.
Relative Energy Brightness versus Color curve for different temperatures Peak Frequency Frequency is proportional to temperature. Frequency f T (measured in Kelvins)
Absorption Spectrum of Hydrogen Discrete Emission Spectrum Slit Discrete Absorption Spectrum Hydrogen Gas Film White Light Source Prism Photographic Film
Absorption Spectra Frequencies of light that represent the correct energy jumps in the atom will be absorbed. When the atom de-excites, it emits the same kinds of frequencies it absorbed. However, this emission can be in any direction.
Close inspection of the absorption spectrum of the sun reveals missing lines known as Fraunhofer lines. In 1868 a pattern of lines was observed in the solar spectrum that represented an element that had not been found on earth. It was Helium named for Helios, the sun.
Doppler shifts are observed in the spectra of stars. If a star is approaching, its spectra will be blue shifted. If a star is moving away, its spectra will be red shifted. Most spectra are red shifted indicating that on the average the universe is expanding.
FLUORESCENCE Some materials that are excited by UV emit visible. These materials are referred to as fluorescent materials.
Fluorescent Lamps Primary excitation - electron collisions with low pressure Hg vapor, UV given off Secondary excitation - UV photons absorbed by phosphors. Phosphors fluoresce emitting visible light. Remember that the visible light from the excited mercury vapor is also emitted.
PHOSPHORESCENCE Electrons get "stuck" in excited states in the atoms and de-excitation occurs at different times for different atoms. A continuous glow occurs for some time. Bioluminescence: Created by chemical reactions inside living organisms.
LASERS Light Amplification by Stimulated Emission of Radiation
Lasers produce coherent light. Coherent light is light with the same frequency and the same phase.
Review Questions
What type of spectrum would you expect to obtain if white light is shined through sodium vapor? (a) an emission spectrum of sodium (b) an absorption spectrum of sodium (c) a continuous spectrum
In fluorescence which has the higher energy, the radiation absorbed or the radiation of an emitted photon? (a) absorbed (b) emitted (c) they both have the same energy
Which phenomenon has electrons getting "stuck" in excited states? (a) incandescence (b) fluorescence (c) phosphorescence
What causes laser light to have all of its waves moving in the same direction? (a) the mirrors in the laser (b) the stimulated emission of the atoms to radiate in the same direction (c) atoms are lined-up in the crystal so that they emit light only in one direction Link to Chapter 27