Light, Plank, Einstein Bohr

8/14/2019 Light, Plank, Einstein Bohr

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Let there beLet there be

LLiigghhtt!!


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Light

The study of light led to the development ofthe quantum mechanical model.

Light is a kind of electromagnetic radiation.

Electromagnetic radiation includes many kinds

of waves.

All waves move at 3.00 x 108

m/s (abbreviatedwith a c) when in a vacuum (actually 2.9979

x 108 m/s).


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Parts of a WaveParts of a Wave

Wavelength

( )

AmplitudeOrigin

Crest

Trough


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FrequencyFrequency

The number of waves that pass a

given point per second.

Units are cycles/sec or hertz (hz) Abbreviated with the Greek letter nu

()


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Frequency & WavelengthFrequency & Wavelength

Are inversely related

As one goes up the other goes down.

c = Different frequencies are different colors of

light. There is a wide variety of frequencies.

The whole range is called the spectrum


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PracticePractice

What is the wavelength of radiationwhose frequency is 1.50x1015 /s?

Is this longer or shorter than red light?

My favorite AM radio station is1150kHz. What is the wavelength of

this station?


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Introducing Mr. Maxwell

Planck


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Max Planck (1858 1947)Max Planck (1858 1947) Prevailing ideas were that matter was

composed of particles and energy in the form

of light was a wave.

Studying radiation profiles emitted by solidbodies heated to incandescence, Plank found

that the results couldNOT BE EXPLAINED

with the assumption that matter could absorbor emit any quantity of energy.

Planck studied Blackbody radiation for the

answers.


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Blackbody RadiationBlackbody Radiation

When a solid or liquid object is heatedthey will emit light. A toaster or oven

heating element or even a tungsten light is

an example. This is blackbody radiation. It was known that the frequencies of

vibrating atoms in a heated object were

responsible for the emitted radiation, but

Classical physics was not able to explain

the distribution of frequencies.


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Quantization

Max Planck observed that energy emitted by hotobjects only had certain wavelengths.

Instead of light of any energy, only

photons at integer multiples ofa small packet of energy ever

were found.

This minimum packet size was found to be6.626 x 10-34 Js,

which is now known as

Plancks constant and

has the symbol, h.

E=nh


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Introducing Mr. Albert

Einstein


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The Photoelectric EffectThe Photoelectric Effect

It was determined experimentally that lightshining on a metal surface will eject electrons

from the metal surface.

The results will differ as to whether or not light

is a particle or a wave, which we will see in a

minute.


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Light, although thought of as a wave, didnt behave asif it was a wave in this experiment.

A photoemissive surface would release electrons only

if the light was at or above a specific minimumfrequency.

The intensity or strength of the incident light did not

affect the maximum kinetic energy of the emitted

photoelectrons. However, the rate at which

photoelectrons were released did depend on the

intensity of light.

h h l i


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Even with increased

intensity

The PhotoelectricThe Photoelectric

EffectEffectEvidence for the quantization of light can beseen in the photo electric effect.

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Low energy light

produces no effect.High energy light

ejects an electron

Higher frequency

ejects higher energy

electrons


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The maximum kinetic energy of the emittedphotoelectrons depended on the frequency of

the incident light (provided it was above the

minimum threshold frequency). None of these behaviors could be explained by

classical physics descriptions of light as a

wave.


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1955)1955) Albert Einstein proposed that electromagnetic

radiation can be viewed as a stream ofparticles known as photons.

Albert Einstein actually won the Nobel Prize

for his work on the photoelectric effect.

In 1913 to 1914, R.A. Milliken did a series ofexperiments that proved all of Einsteins

theories!


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To conclude the photoelectric effect phenomenonif light

was composed of photons whose energy was equal to hh,

then the energy level being dependent on frequency would

make sense of the whole thing!

Higher frequency photons have more energy, so they should

make the electrons come flying out faster. In other words,

more energy would be transferred to them.

Using higher intensity light should cause more electrons tocome flying out, and using very low frequency light then

none of the photons will have enough energy to knock out

an electron.


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Mr. Einstein, meet Mr.Planck

E=mc2 E=nh

Ephoton=hm = E

c2 Ephoton=hc

c=

= ___c


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Does this mean that photons have mass?

yes, in some sense, but only when it is moving

But isnt light a wave?

yes, some times it is

m = E

c2Ephoton=hc

m = hc/c2

m = hc

Huh?


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Wave/Particle Duality

Some times you feel like a nut, sometimes you dont.

Wave-Particle Duality
http://hyperphysics.phy-astr.gsu.edu/hbase/mod1.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/mod1.html


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AtomicAtomicSpectrumSpectrum

So how does color tell us aboutSo how does color tell us aboutatoms, I mean really, get on withatoms, I mean really, get on with

it! XDit! XD


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A PrismA Prism White light is made up

of all the colors of the

visible spectrum.

Passing it through a

prism separates it

because the different

wavelengths bend at

different angles whenentering and leaving the

substance.


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If the light is not whiteIf the light is not white

By heating an element withelectricity or heat, a characteristic

color of light is given off. When

this light is passed through a

spectrum, there are distinct

wavelengths of light present and

nothing in between!

This unique fingerprint, can beused to identify elements present in

suns, other planets, unknown

samples, etc.

(H )


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mercury (Hg)

sodium (Na)

neon (Ne)

hi b h i b i lN hi b h i b Ni l


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Nothing bohring about NielsNothing bohring about Niels

BohrBohr

In 1913, Niels Bohr developed a theoreticalexplanation for the observed emission spectra

of the Hydrogen Atom.

Bohr Assumed that electrons were orbiting thenucleus and from classical physics, he

assumed that they traveled in a circular path

much like planets did around the sun.

Bohr borrowed the idea of quantized energy

from our good friend Maxwell Planck.


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Niels BohrNiels Bohr

Bohr proposed that only orbits of certain radii,corresponding to defined energies are

permitted.

An electron orbiting in one of these orbitshas

1. A defined energy state.

2. Will not radiate energy

3. Will not spiral into the nucleus.


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Excited Energy StatesExcited Energy States

If you add energy to an electron it can jump from oneenergy level to another producing an excited state.

1H

Ground StateH: 1st level Excited State

H: 2nd level


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Atomic EmissionAtomic Emission

A photon of light is emitted when the electron returnsto its ground state and releases the previous quantum

amount of energy absorbed.

1H

Ground StateH: 1st level

Excited StateH: 2nd level

There are many energy levelsThere are many energy levels


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There are many energy levelsThere are many energy levels

and many transitions thatand many transitions that

can occurcan occur

1Hn =7 6 5 4 3 2 1

Lyman Series:

(ultraviolet)

Balmer Series:

(visible) Paschen series:

(infrared)


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Review: Bohrs Model

Electrons exist in specific energy levels outside thenucleus, can move from one level to another, andoccupy the lowest energy levels possible understable conditions.

When exposed to energy (electrical current, flame,etc.), electrons absorb a quantum of energy andmove to an excited state. Energy is released in theform of light when the electrons return to theground state.

The further an electron jumps the more energy isabsorbed or released when it returns to its ground

state.


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PropertiesProperties Light has a dual nature. It behaves as both a particle

and a wave. Electromagnetic radiation, which was

previously thought to exhibit only wave properties,

seems to show characteristics of particulate matter

as well and can be considered as a stream ofphotons.

Energy is quantized, it seems to occur only in

discrete units called quanta. Light energy dependson its frequency as shown on the next slide.


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QuantizedThe Quantum of Energy was equal to

E = nhE is the energy of the photon

(nu) is the frequency

h is Plancks constant = 6.626 x 10 -34Joule

secn is an integer (1, 2, 3, etc.)

A Joule is the metric unit of Energy


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Review of JoulesReview of Joules Energy is a force moving through a distance. Force is

equal to mass times acceleration. Therefore Force = ma (mass times acceleration)

Acceleration = meter/s2

Mass is measured in kilograms And so a Force = kilogram meter/s2

Which, incidentally is called aNewton.

Joule = Force times a distance

Therefore, Joule = (kg m2)/s2

And now you have units for Energy!


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The End!!

Light, Plank, Einstein Bohr

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