Physics

Light Emission

Atomic Theory

▪ The Greeks, around 400 BC, were the first to theorize that matter is composed of indivisible units, called atoms.

▪ Democritus

Atomic Theory

▪ J.J. Thomson discovered the electron in 1897.

▪ Evidence that atoms themselves are divisible and are made up of even smaller parts.

▪ Thomson’s model is called the “plum pudding” model because he envisioned negative charges evenly distributed in a positively charged “dough”.

Atomic Theory

▪ Ernest Rutherford performed his famous gold-foil experiment in 1911, leading to the discovery of the atomic nucleus.

Rutherford’s Gold Foil Experiment

Rutherford was surprised to find that some of the alpha particles were deflected at extreme angles – a few nearly reversed direction completely!

• This led to the proposition that the positive region inside gold atoms was not evenly distributed, but rather concentrated in a very tiny nucleus at the center.

Atomic Theory

▪ Rutherford’s model raised many new questions:

▪ How can a nucleus stay together with the large repulsive electric forces that must be acting on the positive charges?

▪ How can an orbiting electron be sustained indefinitely around the nucleus of the atom? Why doesn’t it run out of energy and fall into the positive nucleus?

Atomic Theory

▪ In 1913, Niels Bohr’s suggested a model of the atom in which electrons of atoms could only occupy exact energy levels.

An energy level is a specific energy state with an exact quantum of energy.

Excitation

An electron in a state above its lowest possible state is excited. Returning to is lowest state, it emits radiant energy, a photon.

Light emission involves the transitions of electrons from higher to lower energy states within atoms.

Excitation and De-Excitation

Absorption

Atom in an Excited State

Emission

Excitation and De-Excitation

E ~ f

E = hf

h = 6.63 x 10-34 joule ▪ secondsor

4.14 x 10-15 electron volt ▪ seconds

Energy Levels of an Atom

Partial energy levels of a hydrogen atom

Absorption

Absorption is indicated by upwardly drawn arrows in energy level diagrams.

Emission

When an electron drops to a lower energy level, it loses energy. This energy is given off as a photon of light.

Problem

A) The energy level diagram shown shows a sample of atoms initially in the ground state. The atoms are radiated by photons having 8 eV of energy. Determine the energy level of the electrons after they absorb the 8 eV photons.

0 eV-1 eV-2 eV

-5 eV

-10 eV

n=4n=3

n=2

n=1

B) Subsequently, the resulting excited electrons drop to lower energy levels, emitting photons of light. Determine all the possible energies of the emitted photons.

Energy Relates to Frequency & Wavelength

E = hf and c = f λ

With a little manipulation, we see that:

𝐸 =ℎ𝑐

𝜆

• In a tube of neon (or other gas), electrons are jostled back and forth at high speeds by a high ac voltage.

• These electrons smash into millions of gas atoms, boosting orbital electrons into higher energy levels.

• This energy is then radiated as light when they fall back to their ground states.

A “neon” sign transforms electrical energy into radiant or

light energy.

Excitation

Excitation

The colors of various flames are due to excitation.

Different atoms in the flame emit colors characteristic of their energy-level spacings.

Every element burns with its own characteristic color or colors.

Excitation

Aurora Borealis and Australis are caused by high speed charged particles from the solar wind which strike molecules in the upper atmosphere. They emit light exactly like the neon tube.

Oxygen - Greenish / WhiteNitrogen Molecules – Red / VioletNitrogen Ions – Blue / Violet

There are also UV and X-ray emissions we can’t see.

With atomic excitation / de-excitation we

encounter the first instance where Classical

Physics falls short, and we need a different set

of rules to explain what we observe –

Quantum Mechanics

Classical Physics Breaks Down!

Atomic Spectra

Every element has its own characteristic pattern of electron energy levels and therefore emits light with its

own characteristic pattern of frequencies called its EMISSION SPECTRUM, when excited.

Light is passed through a thin slit and then focused through a prism (or diffraction grating) onto a viewing screen behind.

Spectroscope

Atomic Spectra

Atomic Spectra

Spectral lines are as characteristic of each element as are the fingerprints of people.

Emission Spectra

EMISSION SPECTRUM

ABSORPTION SPECTRUM

Continuous Spectrum

Emission Lines

Absorption Lines

Emission / Absorption Spectra

Absorption Spectra from Stars

Upon close inspection, the sun’s spectrum is NOT continuous because the stars are surrounded by an atmosphere of cooler

gases that absorb some of the light from the main body.

Absorption Spectra

Just as a moving sound source produces a Doppler shift in its pitch, a light source produces a Doppler shift it its light frequency.

Frequency Shift

Almost all the galaxies show a red shift in their spectra –evidence the universe is expanding!

Emission of Light

▪Incandescence

▪Fluorescence

▪Phosphorescence

Incandescence

Light produced as a result of high temperature is called incandescent.

It contains an infinite number of frequencies spread smoothly across the spectrum.

Continuous Spectrum produced by incandescence

Incandescence

Light emitted by atoms far from one another (gas phase) is quite different from the light emitted by the same atoms closely packed together in a solid.

Incandescent light depends on temperature.

Incandescence

Fluorescence

Atoms that undergo excitation when illuminated with UV light sometimes emit visible light upon de-excitation.

That is, they fluoresce.

Fluorescence

Fluorescence

Fluorescence

Fluorescence

PhosphorescencePhosphorescence is similar to fluorescence but with a time-delay before de-excitation occurs. Electrons in higher energy levels become “stuck” temporarily.

Phosphorescence

The element, phosphorous, is used to make things “glow in the dark”.

Phosphorescence

Phosphorescence

Phosphorescence

Phosphorescence

Lamps

INCANDESCENT LAMP

Tungsten filament heats to about 3000 K.

Glass prevents oxygen from reaching the hot filament.

If you add a small amount of a halogen like iodine, the filament will last longer.

1. Electrons from electrodes forced to vibrate to and fro at high

speeds by ac voltage.

FLUORESCENT LAMP

1. Electrons from electrodes forced to vibrate to and fro at high

speeds by ac voltage.

2. Primary Excitation Process: Mercury atoms are excited by

electron impacts and emit UV photons.

FLUORESCENT LAMP

1. Electrons from electrodes forced to vibrate to and fro at high

speeds by ac voltage.

2. Primary Excitation Process: Mercury atoms are excited by

electron impacts and emit UV photons.

3. Secondary Excitation Process: Phosphors which cover the inner

surface of the tube are excited by the UV photons and fluoresce.

FLUORESCENT LAMP

COMPACT FLUORESCENT LAMP (CFL)

A CFL is just a fluorescent lamp in miniature wrapped into a coil shape and able to be plugged into a standard socket.

LIGHT EMITTING DIODE (LED)

LIGHT EMITTING DIODE (LED)

LightAmplification byStimulatedEmission ofRadiation

INCOHERENT LIGHT

Light emitted by a common lamp is incoherent. It spreads out after a short distance, becoming wider and wider and less intense with increased distance.

MONOCHROMATIC, BUT STILL OUT OF PHASE

Now we have filtered the light so we only have single-frequency waves, but it is still incoherent because the waves are out of phase with one another.

COHERENT LIGHT

All waves are identical, in phase, and in the same

direction.

This is the kind of light that makes up a laser

beam – Coherent Light.

Lasers

Lasers

Lasers

Lasers

Lasers