5. The Nature of Light. Does Light Travel Infinitely Fast? EMR Travels At Finite Speed. EMR: Electric & Magnetic Waves

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1 5. The Nature of Light Light travels in vacuum at 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 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 = km/s c = m/s c = 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 vector vibrates synchronized w/e vector

2 Refraction of Sunlight By a Prism The Celebrated Phenomenon of Colours Prisms Do Not Add Color to Light Red light is refracted least Blue light is refracted most 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 = Cold Not looking directly at a hot high-density object Dark-line Cool low-density objects Cool interstellar gas clouds Continuous and Line Spectra Absorption from a cool low density object = + Emission from a hot Emission from a hot high 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 T 4

3 Blackbody Curve: The Ideal Blackbody Curve: The Sun White stars Our Sun Red stars Wien s Law Blackbody radiation curves have one peak This wavelength emits the most energy This wavelength depends on Kelvin temperature! max = T λ max = Wavelength of maximum emission (meters) T = Temperature (kelvins) λ max 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 =! "T 4 F = Energy flux (joules. m 2. sec 1 ) σ = Constant = W. m 2. K 4 T K = Temperature (kelvins) Energy is directly proportional to T K 4 Raising T K 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 λ 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

4 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

5 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 Hydrogen Electron Transitions Electron orbitals are not to scale

6 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 wavelengths Higher frequencies Lower frequencies Shift toward blue Shift toward red Important Concepts Light in vacuum at constant speed 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