Blackbody SpectrumBlackbody Spectrum•Remember that EMR is characterized by wavelength (frequency)

•Spectrum: distribution of wavelength (or frequency) of some EMR

•Blackbody: the distribution of EMR at equilibrium with matter

•Equilibrium means that matter and EMR exchange the same amount of energy

•Blackbody emission is a continuum: all wavelengths are present, although with different intensity

Hotter objects emit photons with a Hotter objects emit photons with a higher average energy.higher average energy.

Wien’s LawWien’s Law

• The peak of the blackbody emission The peak of the blackbody emission spectrum is given by spectrum is given by

nmT(Kelvin)

109.2 6

max

The higher the temperature, the shorter the wavelength, i.e. the bluer

ThermalThermal Radiation Radiation

• Thermal radiation is basically Black-Thermal radiation is basically Black-body radiation, or nearly sobody radiation, or nearly so

• Every object with a temperature Every object with a temperature greater than absolute zero emits greater than absolute zero emits radiation.radiation.

• Hotter objects emit more total radiation Hotter objects emit more total radiation per unit area.per unit area.

• Hotter objects emit photons with a Hotter objects emit photons with a higher average energy.higher average energy.

• Thermal Radiation (BB) is an example Thermal Radiation (BB) is an example of “continuum emission”.of “continuum emission”.

The graph below shows the blackbody spectra of The graph below shows the blackbody spectra of three different stars. Which of the stars is at three different stars. Which of the stars is at the highest temperature?the highest temperature?

1) Star A1) Star A

2) Star B2) Star B

3) Star C3) Star CA

B

C

EnergyperSecond

Wavelength

Hotter objects emit more total Hotter objects emit more total radiation per unit surface area.radiation per unit surface area.

Stefan-Boltzmann Law: Emitted power per unit area = σ T4

σ = 5.7 x 10-8 W/(m2K4)

You are gradually heating up a rock in an oven You are gradually heating up a rock in an oven to an extremely high temperature. As it to an extremely high temperature. As it heats up, the rock emits nearly perfect heats up, the rock emits nearly perfect theoretical blackbody radiation – meaning theoretical blackbody radiation – meaning that itthat it

1) is brightest when hottest.1) is brightest when hottest.2) is bluer when hotter.2) is bluer when hotter.3) is both3) is both4) is neither4) is neither

For star: L=A For star: L=A TT44 , , A is its surface area A is its surface area

How to Make Light (Part 2)How to Make Light (Part 2)

Structure of Structure of atomsatoms

Energy levels Energy levels and transitionsand transitions

Emission and Emission and absorption linesabsorption lines

Light scatteringLight scattering

AtomsAtoms

A Planetary Model of the Atom

Atoms are made of electrons, protons, and neutrons.

The bounding force: the attractive Coulomb(electrical) force between the positively charged nucleus and the negatively charged electrons.

Electrons can be in different orbits of certain energies, called energy levels.

Different atoms have different energy levels, set by quantum physics.

Quantum means discrete!

Energy Levels Energy Levels

To change its energy levels, an electron must either absorb or emit a photon that has the same amount of energy as the difference between the energy levelsE = h = h c/

Larger energy difference means higher frequency.

Different jumps in energy levels means different frequencies of light absorbed.

Excitation of AtomsExcitation of Atoms

Spectral Line EmissionSpectral Line Emission

If a photon of exactly the right energy is absorbed by an electron in an atom, the electron will gain the energy of the photon and jump to an outer, higher energy orbit.

A photon of the same energy is emitted when the electron falls back down to its original orbit.

Spectral Line EmissionSpectral Line Emission

Collisions (like in a hot gas) can also provide electrons with enough energy to change energy levels.

A photon of the same energy is emitted when the electron falls back down to its original orbit.

Energy Levels of a Hydrogen AtomEnergy Levels of a Hydrogen Atom

Different allowed “orbits” or energylevels in a hydrogenatom.

Emission line spectrum

Absorption line spectrum

Spectral Lines of Some ElementsSpectral Lines of Some Elements

 Argon 

                                                                                

 Helium

                                                                                 

 Mercury

                                                                                 

 Sodium

                                                                                 

 Neon

                                                                                 

Spectral lines are like a cosmic barcode system for elements.

Atoms of different elements have unique Atoms of different elements have unique spectral lines because each elementspectral lines because each element

has atoms of a unique colorhas atoms of a unique color has a unique set of neutronshas a unique set of neutrons has a unique set of electron orbitshas a unique set of electron orbits has unique photonshas unique photons

Blackbody radiation

The Solar SpectrumThe Solar Spectrum

There are similar absorption lines in the other regions of the electromagnetic spectrum. Each line exactly corresponds to chemical elements in the stars.

Reflection nebulaReflection nebula

Emission nebulaEmission nebula

An Object’s SpectrumAn Object’s Spectrum

Encoded in an object’s spectrumis information about the emitter/absorber. This is how we learn what the Universeis made of!