Chapter 30 Light Emission Radio waves are produced by electrons moving up and down an antenna. Visible light is produced by electrons changing energy.

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

In this configuration the atom is not excited.

Here comes a photon

In this configuration the atom is not excited.

e-

e-

Now the atom is said to be excited becausethe electron is in a higher than normal orbit.

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 ExcitationAtomic 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

Spectra of Some Gases – Next Slide Spectra of Some Gases – Next Slide

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

and Sodium Vapor Lamp and Sodium Vapor Lamp

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) f T

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 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.

3. FLUORESCENCE

Some materials that are excited by UV emit

visible.

These materials are referred to as fluorescent

materials.

Demo – Fluorescence with Green Laser and Demo – Fluorescence with Green Laser and

Liquids Liquids

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. PHOSPHORESCENCEElectrons 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 Objects

(including helicopter and (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 Laser - Helium-Neon Laser

LasersLasers

Chapter 30 Review QuestionsChapter 30 Review Questions

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

(a) an emission spectrum of sodium

(b) an absorption spectrum of sodium

(c) a continuous spectrum

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

(a) absorbed

(b) emitted

(c) they both have the same energy

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

(a) incandescence

(b) fluorescence

(c) phosphorescence

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