Chapter 30

Light Emission

Radio waves are produced byelectrons moving up and down an antenna.

Visible light is produced byelectrons changing energy states in an atom.

1. 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).

e-

Here comes a nucleusWith possible orbits for electrons

e-

e-

Here comes a photon

Now the atom is said to be excited becausethe electron is in a higher than normal orbit.In this configuration the atom is not excited.Now the atom is in a non-excited state again.It emitted a photon when it changed orbits.

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.

Video - Atomic ExcitationVideo - Atomic Excitation

This emitted radiation gives the

characteristic colors of the element

involved.

The atoms do not “wear out.”

Demo - Flame ColorsDemo - Flame Colors

Classical explanation fails - it says

there should be radiation even when

there has been no change in energy

levels.

The electron should spiral into the

nucleus - the ultraviolet

catastrophe.

E = hf (h = Planck’s constant)

Emission SpectraEmission Spectra

Continuous Emission Spectrum

Prism

Photographic Film

Slit

White Light Source

Emission Spectra of Emission Spectra of HydrogenHydrogen

Prism

Photographic Film

Film

Slit

Low DensityGlowing

Hydrogen Gas

Discrete Emission Spectrum

Slide - Spectra of Some GasesSlide - Spectra of Some Gases

Demo - Spectra of a Discharge Tube Gas Demo - Spectra of a Discharge Tube Gas

2. INCANDESCENCE

Demo - Spectrum of Incandescent BulbDemo - Spectrum of Incandescent Bulb

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.

Frequency

Rel

ativ

e E

ner

gy

(measured in Kelvins) Tf

Brightness versus color curve for different

temperatures

Peak Frequency

Discrete Absorption Spectrum

Absorption Spectraof Hydrogen

Prism

Photographic Film

Film

Slit

White Light Source

Discrete Emission Spectrum

Hydrogen Gas

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

wasHelium - 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.

3. 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.

4. 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.

Demo - Fluorescent & Phosphorescent Demo - Fluorescent & Phosphorescent

Objects (including helicopter and Objects (including helicopter and

“ “Starry, Starry Night”)Starry, Starry Night”)

Bioluminescence

Light

Amplification by

Stimulated

Emission of

Radiation

E5. LASERSLAS R

Lasers produce coherent light.

Coherent light is light with the same

frequency and the same phase.

Demo - Making Laser Beam VisibleDemo - Making Laser Beam Visible

Early model - ruby crystal doped with Cr

Slide - Helium-Neon LaserSlide - Helium-Neon Laser

Video - LasersVideo - Lasers

Today's models include gas (He-Ne), glass,

chemical, liquid, semiconductor, and x-ray.

Applications:

Surveying Surgery Communication Computer storage Pollution detection 3-d TV (holograms) Thermonuclear fusion - heat H2 pellets

Chapter 30 Review QuestionsChapter 30 Review Questions

In fluorescence which has the In fluorescence which has the higher energy, higher energy, the radiation radiation absorbed or the radiation of an absorbed or the radiation of an emitted photon?emitted photon?

(a) absorbed

(b) emitted

(c) they both have the same energy

(a) absorbed

What type of spectrum would you What type of spectrum would you expect to obtain if white light is expect to obtain if white light is shined through sodium vapor?shined through sodium vapor?

(a) an emission spectrum of sodium

(b) an absorption spectrum of sodium

(c) a continuous spectrum

(b) an absorption spectrum of sodium

What causes laser light to have What causes laser light to have all of its waves moving in the all of its waves moving in the same direction?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

(a) the mirrors in the laser

Each element gives off a Each element gives off a characteristic color when it characteristic color when it undergoes the excitation and de-undergoes the excitation and de-excitation process. The color is excitation process. The color is due todue to(a) fluorescence

(b) secondary emission

(c) electrons moving to lower energy states

(d) refraction of light through the characteristic gas

(e) electrons giving off Cerenkov radiation

(c) electrons moving to lower energy states

Which phenomenon has Which phenomenon has electrons getting "stuck" in electrons getting "stuck" in excited states?excited states?

(a) incandescence

(b) fluorescence

(c) phosphorescence

(c) phosphorescence