Light Emission Chapter 30

Dispersive Element Basic Spectrograph Collimating Lens Recording Device Slit Imaging Lens

Mercury near Horizon The atmosphere can act like a prism

Continuous Spectrum Source must be HOT and DENSE

Emisson (Bright Line) Spectrum Source must be HOT and TENUOUS

Absorption Spectrum

Types of Spectra Continuous Emission Absorption

Rutherford Scattering Experiments Atom He “bullet”

Hydrogen Atom u Simplest atom u One proton, one electron u Most abundant atom u 90% of the universe is hydrogen

Planetary Model Force p e-e- Since electron orbits the proton, a force exists.

Planetary Model p e-e- Acceleration Force implies acceleration Accelerating charges emit light Light carries energy (E = hf)

Planetary Model p e-e- Electron moves closer to the nucleus since it requires less energy to be there.

Planetary Model u But the electron is still accelerating u Must still be radiating energy (light) u Must move still closer to the nucleus u Electron will spiral into and collide with the nucleus (in about seconds) Atoms do not exist!!

Bohr’s Hypotheses u Stable electron orbits exist where the electron does not lose energy.

Electrons can be here or here but not here p

Bohr’s Hypotheses u Transitions can occur between orbits so long as the electron ends up with the energy of the new level.

Hydrogen EnergyEnergy Å photon Absorption

Hydrogen EnergyEnergy Å photon Emission

+P nm 700 nm 400 nm -e Hydrogen Atom 1 -e -e 5 -e 486 nm 434 nm -e 410 nm

Emission and Absorption Lines u Lines come from electron transitions u Energy change either comes from (absorption) or is given to (emission) photon. E u Photon energy  Frequency u E = hf u Frequency  1/(Wavelength)  f = c/ u Wavelength means COLOR

Hydrogen Spectrum EnergyEnergy Lyman (UV) Balmer (VIS) Paschen (IR) Brackett (Far IR)

Aluminum Oxygen Argon Calcium Carbon Helium Hydrogen Iron Krypton Magnesium Neon Nitrogen Sulfur Sodium Xenon

Atomic Spectra Review

Continuous Radiation u How bright is the continuous spectrum at different colors? u How does a perfect light source emit its light? 400nm 500nm 600nm 700nm

Early Experiments Darken inside with carbon black Measure radiation that emerges from hole Blackbody Radiation

Vis IR UV

Changes with Temperature 7000 K 6000 K 5000 K * * * Visible

Blackbody Radiation u Everything in nature emits light u The type of light emitted depends on the temperature of the source u The walls of this room u IR u Stars u Visible and UV u The Corona of the Sun (2,000,000 K) u X-rays

Wien’s Displacement Law u The wavelength of the peak of the blackbody curve is inversely proportional to the temperature. Cool stars are red - hot stars are blue max  1/T f max  T

Fluorescence UV Photon Visible Photon

Phosphorescence UV Photon Visible Photon Let’s wait here

LASERS Light Amplification by Stimulated Emission of Radiation Monochromatic: one specific wavelength (color) of light Coherent: every wave is moving in step Directional: beam is tight, strong and concentrated

“Pumping”