5. The Nature of Light• Light travels in vacuum at 3.0 . 108 m/s• Light is one form of electromagnetic radiation• Continuous radiation: Based on temperature• Wien’s Law & the Stefan-Boltzmann Law• Light has both wave & particle properties• Each element has unique spectral lines• Atoms: A nucleus surrounded by electrons• Spectral lines: Electrons change energy levels• Spectral lines shift wavelength due to motion

Does Light Travel Infinitely Fast?• Some ancient common experiences

– Lightning & thunder• At minimum, light travels faster than easily

measured• At maximum, light might travel infinitely fast

– Galileo’s experiments• Human reflexes are much too slow• Human pulse is much too long

• Olaus Rømer 1676– Inconsistencies in occultations of Jupiter’s moons

• Earlier than expected with Jupiter closer than average• Later than expected with Jupiter farther than average

EMR Travels At Finite Speed

Occultation

Occultation

Light Moves in Vacuum 3.0 . 108 m/s• Light travels at constant speed in vacuum

– Recognized by Einstein as highest possible speed– Independent of the speed of any observer– That speed is…c…and is… “celeritas”

c = 3.0 . 105 km/sc = 3.0 . 108 m/sc = 3.0 . 1010 cm/s

• Light travels different speeds in different media– Air slows light a little Low density

• Light bends/refracts a little as it enters the atmosphere

– Glass slows light a lot High density• Light bends/refracts a lot as it enters a telescope

lens

“Light” is Electromagnetic Radiation• “Light” is one form of electromagnetic radiation

– Electric & magnetic components are sine waves• Electric & magnetic components identical wavelengths• Electric & magnetic components perfectly synchronized

• Various regions electromagnetic radiation– R Radio Longest λ’s Low energies– I Infrared– V Visible “Light” Medium

energies– U Ultraviolet– X X-ray– G Gamma-ray Shortest λ’s High energies

EMR: Electric & Magnetic Waves

• Wave properties– Electric vector vibrates in a sine wave form

vibrates in a single plane– Magnetic vector vibrates in a sine wave form

vibrates perpendicular to e– vectorvibrates synchronized w/e– vector

Refraction of Sunlight By a PrismThe “Celebrated Phenomenon of Colours”

Red light isrefracted least

Blue light isrefracted most

Prisms Do Not Add Color to Light

• Newton’s prism experiments– Isolate one color from sunlight using one prism– Pass that color through a second prism

• No color is added

The Electromagnetic Spectrum

Emission & Absorption Spectra• Emission spectra Bright = Hot

Looking directly at a hot high-density object– Continuous Hot high-density objects

• Hot stars with no intervening interstellar gas clouds– Bright-line Hot low-density objects

• Hot interstellar gas clouds between any star & the Earth

• Absorption spectra Dark = ColdNot looking directly at a hot high-density object– Dark-line Cool low-density objects

• Cool interstellar gas clouds

Continuous and Line SpectraAbsorption from a coollow density object

Emission from a hot Emission from a hothigh density object low density object

=+

The Blackbody Concept• Blackbody: An ideal concept

– Absorbs 100% of all wavelengths of incident EMR• All X-rays, visible light, radio waves…• Experience shows that this is impossible

– Emits all absorbed energy as blackbody radiation• Radiation based exclusively on Kelvin temperature• Experience shows that this actually happens

• Wien’s Law– Wavelength at which the most energy is produced

• Stefan-Boltzmann Law– Total energy is proportional to T4

Blackbody Curve: The Ideal

“White” stars

Our Sun

“Red” stars

Blackbody Curve: The Sun

Wien’s Law• Blackbody radiation curves have one peak

– This wavelength emits the most energy– This wavelength depends on Kelvin temperature

lmax = Wavelength of maximum emission(meters)

T = Temperature (kelvins)

• lmax is inversely proportional to Kelvin temp.– Higher temperature Shorter wavelength

The Stefan-Boltzmann Law• Blackbody radiation curves show energy flux

– This energy flux depends on Kelvin temperature

F = Energy flux (joules . m–2 . sec–1 )s = Constant = 5.67 . 10–8 W . m–2 .

K–4

TK = Temperature (kelvins)

• Energy is directly proportional to TK4

– Raising TK by a factor of 10 raises energy by 10,000

The Wave-Particle Nature of EMR• EMR behavior depends on the experiment

– Wave experiment: EMR behaves like a wave• Young’s double-slit experiment

– Particle experiment: EMR behaves like a particle• EMR as photons

– A quantum amount of EMR energy– Energy = Planck’s Constant . Frequency

• The photoelectric effect– Electron emission requires some minimum energy

• Possible only if photons actually exist

Each Element Has a Unique Spectrum• Every material has a unique spectral signature

– Unique set of spectral lines• When hot, the spectral lines are bright• When cool, the spectral lines are dark

– Each spectral line has a unique l Spectroscopy– Each spectral line emits a unique amount of energy

• Kirchhoff’s Laws– Hot opaque objects: Continuous spectra

• Classical blackbody radiation– Hot transparent objects: Bright-line spectra

• Hot interstellar gas clouds with no continuous background– Cool transparent objects: Dark-line spectra

• Cool interstellar gas clouds with a continuous background

The Periodic Table of the Elements

Spectra: The Hydrogen Family

Spectra: The Helium Family

Spectra: The Beryllium Family

Spectra: The Boron Family

Spectra: The Carbon Family

Spectra: The Nitrogen Family

Spectra: The Oxygen Family

Spectra: The Fluorine Family

The Bohr Model of the Atom• A central nucleus

– One or more protons Atomic number• Determines the chemical properties (elements)

– Zero or more neutrons Mass number• Determines the nuclear properties (isotopes)

• Electron orbitals surround the nucleus– Neutral atoms: Number of p+ = Number of e–

– Ionized atoms: Number of p+ ≠ Number of e–

• Cations: One or more e– lost Net positive charge

• Anions: One or more e– gained Net negative charge

Bohr Model of the Hydrogen Atom

Electron orbitals are not to scale

Hydrogen Electron Transitions

Electrons Jump Energy Levels• Electrons jumping energy levels produce lines

– Hydrogen atom is the simplest of all• Lyman series: Ultraviolet

spectrum• Balmer series: Visible

spectrum• Paschen series: Infrared

spectrum– All other atoms & elements are more complicated

• More considerations about spectral lines– Each line has a different amount of energy

• Energy = Planck’s constant . Frequency– Each line has a different probability of jumping

• More jumps More energy emitted Brighter lines

Spectra: Hydrogen Energy Levels

The Doppler Effect• Effect Wavelength shift due to relative motion

– Source & viewer moving closer Blue shift• Spectral lines shifted toward blue end of the spectrum

– The spectral lines do not actually appear blue ! ! !– Source & viewer moving farther Red shift

• Spectral lines shifted toward red end of the spectrum– The spectral lines do not actually appear red ! ! !

• Cause Relative motion of source & observer– Source & viewer moving closer

• Waves compressed Shorter wavelength Blue shift– Source & viewer moving farther

• Waves stretched Longer wavelength Red shift

Doppler Shift: Stretching Waves

Compressed wavelengths Stretched wavelengthsHigher frequencies Lower frequenciesShift toward blue Shift toward red

• Light in vacuum at constant speed– 3.0 . 108 m . sec–2

• Light in other media moves slower– Related generally to media density

• Light is one form of EMR– Gamma rays– X-rays– Ultraviolet– Visible– Infrared– Microwave / Radio

• Emission & absorption spectra– Continuous Hot high

density– Bright line Hot low

density– Dark line Cool low

density• Blackbody concept

– Absorbs 100% of all wavelengths– Emits 100% at specific wavelengths

• Wien’s Law– Wavelength of maximum energy

• Stefan-Boltzmann Law– Total energy produced

• Wave-particle duality of all EMR– Behavior depends on experiment– Photoelectric effect

• Unique sets of spectral lines– Kirchhoff’s three laws

• Bohr’s mode of hydrogen– Nucleus with orbitals– Neutral & ionized atoms– Electron energy jumps produce lines

• Doppler effect– Relative convergence: Blue shift– Relative divergence: Red shift

Important Concepts