Interaction of Light with Matter (1900) in the early 1900’s, there were three key behaviors associated with the interaction of light with matter that could

Interaction of Light with Matter (1900)

in the early 1900’s, there were three key behaviors associated with the interaction of light with matter that could not be explained…

1. the spectrum of radiation from a glowing hot object (blackbody radiation – basis of infrared goggles)

2. the emission of electrons from a metal when irradiated by light (photoelectric effect- now the basis of solar panels)

3. the absorption and emission of radiation by matter (color)

we are going to briefly look at the issues andconclusions around the spectrum of radiation from a glowing hot object

Thermal Radiation - Intensity vs Wavelength

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5000 K

3000 K lamp filament)

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6000 K (surface of sun)


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1500 K (red hot iron)

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As an object gets hotter, the spectrum of light given off shifts towards _______________As an object gets cooler, the spectrum of light given off shifts towards _______________Intensity goes ______________ as the temperature goes down.

shorter wavelengthslonger wavelengths

down

Max Karl Ernst Ludwig Planck and

The Particle Nature of Light (1900)

in an effort to explain the radiation given off by hot objects,Planck made a revolutionary suggestion

that electromagnetic radiation comes in quanta

discrete amounts“little packets”pieces

the energy of each “packet”

Epacket = h

Max Karl Ernst Ludwig Planck, (April 23, 1858 – October 4, 1947) was a German physicist who is regarded as the founder of the quantum theory, for which he received the Nobel Prize in Physics

Max Karl Ernst Ludwig Planck and The Particle Nature of Light (1900)

light behaves as if it comes in packets

light (radiation) is not continuousrevolutionary

we perceive the world as continuous - matter in space, timewe sense that all places are possible as we move through space at any time

…can have any energy

revolutionary to suggest that radiation (energy) behaves in a discontinuous way

light (radiation) can behave as waves

as particles Epacket = h

c =

Planck’s constant 6.62610-34 J·sec

there was plenty

of evidence that

light behaves as

as wave

continuous

discrete

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the classical explanation for thermal radiation was based in:- charged objects in matter vibrate- thermally agitated charges can vibrate at any frequency- these charged oscillators emit radiation of that frequency- if the particles can oscillate at any frequency, then any frequency of radiation can be emitted

accounts for continuous spectrum

BUT using this model, the shape of the distribution could NOT be accounted for

Hotter –

vibrate faste

r,

higher f

requency

(s

horter w

avelength)


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tyPlanck produced an empirical equation that fit the data

mathematical contrivanceconcocted by “happy guesswork”

OED- information developed by

observation and experiment,

rather than by deduction

from theory

- guided by mere experience

without scientific knowledge

Planck then set out to develop a theory to explain his equationusing the classical model that oscillators could vibrate at any frequency(therefore emit radiation at any frequency)

he could NOT derive his equation theoretically

so,he made a hypothesis that the emitted energy had to be thought of as

“energy elements” or particles of energy

based in part on work by James Clerk Maxwell and Ludwig Boltzmann (1860-1868) describing the distribution of particle speeds at thermal equilibrium (@ a given T)

………..

James Clerk Maxwell(Irishish)

Ludwig Boltzmann(Austrian)

work by James Clerk Maxwell and Ludwig Boltzman describing the distribution of particle speeds at thermal equilibrium (@ a particular T)

- it was known that if a set of molecules with any initial distribution of energies (speeds) was put into a constant temperature chamber, it would eventually redistribute into the Maxwell-Boltzman distribution at thermal equilibrium- the theoretical explanation for this distribution relied on being able to count the particles

James Clerk Maxwell and Ludwig Boltzman Distribution of particle speeds at thermal equilibrium

Speed Distribution for Water at variable T

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Speed Distribution for Water at variable T

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Shapes similarEquations similar

-to apply the ideas of Maxwell and Boltzman for the theory describing the distribution of speeds of discrete particles at thermal equilibrium to the distribution of thermal radiation,

Planck had to be able to “count” his energy - which means that it (the energy) had to come in discrete countable pieces- the energy of each energy element was proportional to the frequency of the

oscillator

- one of the terms that appeared already in his empirical formula

…back to Planck…

Epacket = h

Planck’s idea that radiant energy came in packets - particles of light - was not based in any physical evidence at the time.

It was a mathematical convenience to consider radiant energy as “energy elements”.It was purely an assumption that allowed him to theoretically model the distributions of thermal radiation.

***** He himself did not believe or accept the particle nature of light.

In 1905, at the age of 23 (?) working as a technical assistant in the Swiss Patent Office in Bern, Switzerland - Albert Einstein - demonstrated the particle nature of light in his explanation of the photoelectric effect.

Einstein was awarded the 1921 Nobel Prize in Physics for

"for his services to Theoretical Physics, and especially for his discovery of the law of thephotoelectric effect"

NOTE: the word “photon” for aparticle of light was coined in 1926 byG.N. Lewis

NOTE: the word “photon” for aparticle of light was coined in 1926 byG.N. Lewis

Albert Einstein (hey! that name is familiar)

Interaction of Light with Matter (1900)

in the early 1900’s, there were three key behaviors associated with the interaction of light with matter that could not be explained…

1. the spectrum of radiation from a glowing hot object (blackbody radiation – basis of infrared goggles)

2. the emission of electrons from a metal when irradiated by light (photoelectric effect- now the basis of solar panels)

3. the absorption and emission of radiation by matter (color)

Flaming Colors!

Our next step towards understanding the structure of the atomis to investigate the interaction of light with matter…. COLOR


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Documents

Interaction of Light with Matter (1900) in the early 1900’s, there were three key behaviors associated with the interaction of light with matter that could