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).

e-

Here comes a nucleus With 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-

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

Prism

Photographic Film

Slit

White Light

Source

Emission Spectrum

of Hydrogen

Prism

Photographic Film

Film

Slit

Low

Density

Glowing

Hydrogen

Gas

Discrete Emission Spectrum

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.

Frequency

Re

lative

En

erg

y

(measured in Kelvins) f T

Brightness versus Color curve for different

temperatures

Peak Frequency

Frequency is

proportional to

temperature.

Discrete Absorption Spectrum

Absorption Spectrum

of 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.

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.

Light

Amplification by

Stimulated

Emission of

Radiation

LASERS

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