© sierra college astronomy department 1 light and matter

© Sierra College Astronomy Department© Sierra College Astronomy Department 11

Light and MatterLight and Matter



IntroductionIntroduction

Astronomy – An Observation Based ScienceAstronomy – An Observation Based Science In situ measurements by spacecraft are extremely In situ measurements by spacecraft are extremely

limited for studying the objects of the cosmoslimited for studying the objects of the cosmos Vast majority of our understanding of the Universe Vast majority of our understanding of the Universe

comes from:comes from:The acquisition of light with a variety of instrumentsThe acquisition of light with a variety of instrumentsThe application of physical principles discovered The application of physical principles discovered

and refined locally and assumed universally valid and refined locally and assumed universally valid to interpret the acquired lightto interpret the acquired light



A First GlanceA First Glance

How Do We Experience Light?How Do We Experience Light? Energy and PowerEnergy and Power

Example: The warmth of the daytime Sun – a form of Example: The warmth of the daytime Sun – a form of radiativeradiative energy energy

UnitsUnits Unit of energy: Unit of energy: joulejoule Unit of energy rate (Unit of energy rate (powerpower): ): wattwatt = 1 joule/sec = 1 joule/sec A human’s 10,000,000 joules/day is about 100 wattsA human’s 10,000,000 joules/day is about 100 watts

Light and ColorLight and Color Light split by a prism into colors gives a Light split by a prism into colors gives a spectrumspectrum All the colors (red, orange, yellow, green, blue, and All the colors (red, orange, yellow, green, blue, and

violet) is roughly equal proportions gives violet) is roughly equal proportions gives white lightwhite light BlackBlack is the lack of color is the lack of color




How Do Light and Matter InteractHow Do Light and Matter Interact Interaction occurs in four basic ways:Interaction occurs in four basic ways:

EmissionEmission – The process by which an object creates and then – The process by which an object creates and then emits light (e.g., the filament of a light bulb)emits light (e.g., the filament of a light bulb)

AbsorptionAbsorption – The process by which an object destroys light by – The process by which an object destroys light by absorbing it (e.g., some of the light from the bulb hits your hand, absorbing it (e.g., some of the light from the bulb hits your hand, is destroyed, and results in the warmth you feel)is destroyed, and results in the warmth you feel)

TransmissionTransmission – Light is neither created or destroyed by an object – Light is neither created or destroyed by an object but simply passes through (e.g., that portion of the bulb light that but simply passes through (e.g., that portion of the bulb light that passes through a window)passes through a window)

Reflection/scatteringReflection/scattering – Light that bounces off matter leading to – Light that bounces off matter leading to what is called what is called reflectionreflection (when the bouncing is all in the same (when the bouncing is all in the same general direction) and general direction) and scatteringscattering (when the bouncing is in (when the bouncing is in random directions)random directions)




How Do Light and Matter InteractHow Do Light and Matter Interact (continued) (continued)

More terminologyMore terminology Materials that transmit light are said to be Materials that transmit light are said to be transparenttransparent Materials that absorb light are said to be Materials that absorb light are said to be opaqueopaque

Be carefulBe careful Some materials can be partially transparent (partially opaque) Some materials can be partially transparent (partially opaque)

and to various degrees (e.g., sunglasses)and to various degrees (e.g., sunglasses) Different colors may behave differently with the same material Different colors may behave differently with the same material

(e.g., green grass absorbs most colors, but reflects/scatters (e.g., green grass absorbs most colors, but reflects/scatters green light)green light)



Properties of LightProperties of Light

Particle vs WaveParticle vs Wave A A particleparticle is a physical object that can be considered is a physical object that can be considered

localized in space while characterized by certain localized in space while characterized by certain measurable properties (e.g., a charged electron or a measurable properties (e.g., a charged electron or a baseball)baseball)

A A wavewave is a spatial and/or temporal variation of a certain is a spatial and/or temporal variation of a certain property or substance that is generally associated with an property or substance that is generally associated with an extended volume (e.g., waves on the surface of a pond)extended volume (e.g., waves on the surface of a pond)

Light exhibits both a particle and wave nature – this Light exhibits both a particle and wave nature – this seemingly contradictory observation is known as the seemingly contradictory observation is known as the wave-particle dualitywave-particle duality



The Wave Nature of LightThe Wave Nature of Light

Wave Motion in GeneralWave Motion in General WavelengthWavelength (()) is the distance from a point on is the distance from a point on

a wave to the next corresponding point.a wave to the next corresponding point. FrequencyFrequency ((f or f or ) is the number of repetitions ) is the number of repetitions

per unit time and often is given in per unit time and often is given in cycles/second or cycles/second or hertzhertz ((HzHz).).

There is a relationship between f and There is a relationship between f and !!



The Wave Nature of LightThe Wave Nature of LightLight as a WaveLight as a Wave White light is made up of light of many White light is made up of light of many

wavelengths, but all wavelengths travel wavelengths, but all wavelengths travel at 300 million meters/second (3.00 X10at 300 million meters/second (3.00 X1088 m/s) m/s) in a vacuumin a vacuum..

f f = c = speed of light wave = c = speed of light wave

NanometerNanometer (nm): (nm): unit of length = 10 unit of length = 10-9-9 m.m.

AngstromAngstrom (Å): (Å): unit of length = 10 unit of length = 10-10-10 m; m; it is a non-SI unit. it is a non-SI unit.



The Wave Nature of LightThe Wave Nature of Light 700 nm red light has 700 nm red light has f f = 4.3 X 10= 4.3 X 101414 Hz. Hz.

400 nm violet light has 400 nm violet light has ff = 7.5 X 10 = 7.5 X 101414 Hz Hz Frequencies range from 10Frequencies range from 1022 Hz (low) to 10 Hz (low) to 102424 Hz (high) Hz (high) Wavelengths range from 10Wavelengths range from 1066 m (long) to 10 m (long) to 10-16-16 m (short) m (short)

Maxwell

Based on frequency and/or wavelength, the Based on frequency and/or wavelength, the Electromagnetic (EM) spectrumElectromagnetic (EM) spectrum is usually is usually broken into these regions: radio broken into these regions: radio (AM/FM/microwave), infrared, visible, (AM/FM/microwave), infrared, visible, ultraviolet, X-ray, gamma rayultraviolet, X-ray, gamma ray

These waves are called “These waves are called “electromagneticelectromagnetic” ” because they consist of combined electric because they consist of combined electric and magnetic waves that result when a and magnetic waves that result when a charged particle acceleratescharged particle accelerates



The PhotonThe PhotonLight as a ParticleLight as a ParticleLight can also be thought of as a Light can also be thought of as a

energy particle or packet.energy particle or packet.This was the conclusion to explain the This was the conclusion to explain the

photoelectric effectEquation for energy of a Equation for energy of a photonphoton::

EE = = hf = hc/hf = hc/

where where EE is the energy of the photon, is the energy of the photon, hh is is Planck’s constantPlanck’s constant, , ff is the frequency is the frequency of the light, of the light, the wavelength and the wavelength and cc is is the speed of light.the speed of light.



The Structure of MatterThe Structure of Matter

Back to the GreeksBack to the GreeksDemocritus (c. 470-380 B.C.) Democritus (c. 470-380 B.C.)

introduced the idea of the introduced the idea of the atomatom – – the smallest possible unit of any the smallest possible unit of any objectobject

He then hypothesized four He then hypothesized four different types of atoms called different types of atoms called elementselements, each of which had , each of which had different shapes and propertiesdifferent shapes and properties



The Structure of MatterThe Structure of MatterBasic Atomic Structure & TerminologyBasic Atomic Structure & TerminologyParts of an AtomParts of an Atom

Protons Protons and and neutrons neutrons are in theare in the nucleus nucleusElectrons Electrons in ain a “cloud” surround the nucleus“cloud” surround the nucleusElectric chargeElectric charge (positive and negative) and (positive and negative) and

forceforceAtomic VariationsAtomic Variations

Different Different elementselements are atoms with different are atoms with different atomic numberatomic number (number of protons in nucleus) (number of protons in nucleus)

Different Different isotopesisotopes of an element have different of an element have different number of neutrons in the nucleus (number of neutrons in the nucleus (atomic atomic mass numbermass number = # of protons + # of neutrons) = # of protons + # of neutrons)



The Bohr AtomThe Bohr AtomThree postulates of the Three postulates of the Bohr atomBohr atom::1.1. Electrons in orbit around a nucleus can Electrons in orbit around a nucleus can

have only certain specific energies have only certain specific energies (these (these energy levelsenergy levels are said to be are said to be quantizedquantized))

2.2. An electron can move from one energy An electron can move from one energy level to another changing the energy level to another changing the energy of the atomof the atom

3.3. This change in energy of the atom is This change in energy of the atom is associated with a photon of equivalent associated with a photon of equivalent energy, which in turn determines the energy, which in turn determines the frequency (or wavelength) of the frequency (or wavelength) of the photonphoton[Note: This change in energy may also be associated [Note: This change in energy may also be associated with other processes – for example, atomic collisions]with other processes – for example, atomic collisions]


Light and MatterLight and MatterThe Bohr AtomThe Bohr Atom

““States” of an electron:States” of an electron: An electron at it minimum energy level is said An electron at it minimum energy level is said

to be in its to be in its ground stateground state If a photon with just the right amount of If a photon with just the right amount of

energy interacts with the atom, the electron energy interacts with the atom, the electron may be raised to a new level; the electron is may be raised to a new level; the electron is said to be in an said to be in an excited stateexcited state..

While in an excited state, the electron can While in an excited state, the electron can “relax” and fall down to a lower energy state “relax” and fall down to a lower energy state releasing a photon.releasing a photon.

Recall: Recall: EE = = hfhf = = hchc//



Spectra Examined Close UpSpectra Examined Close Up

Continuous spectrumContinuous spectrum Contains an entire range of Contains an entire range of

wavelengths rather than wavelengths rather than separate, discrete wavelengths.separate, discrete wavelengths.

Example: The heated filament Example: The heated filament of a lamp or a glowing piece of of a lamp or a glowing piece of iron in the blacksmith’s forge.iron in the blacksmith’s forge.




Kirchhoff’s Laws - BackgroundKirchhoff’s Laws - Background In 1814 Fraunhofer analyzed the solar In 1814 Fraunhofer analyzed the solar

spectrum and found a number of dark spectrum and found a number of dark lines across the continuous spectrum. lines across the continuous spectrum. The dark lines are caused by The dark lines are caused by absorption.absorption.

Later it was discovered that if gases are Later it was discovered that if gases are heated until they emit light, a spectrum heated until they emit light, a spectrum made up of bright lines appears.made up of bright lines appears.




Kirchhoff’s lawsKirchhoff’s laws summarize how the summarize how the three types of spectra are produced:three types of spectra are produced:1.1. A hot, dense glowing object (a solid or A hot, dense glowing object (a solid or

dense gas) emits a dense gas) emits a continuouscontinuous spectrum. spectrum.

2.2. A hot, low-density gas emits light of only A hot, low-density gas emits light of only certain wavelengths - a certain wavelengths - a bright linebright line spectrum.spectrum.

3.3. When light having a continuous spectrum When light having a continuous spectrum passes through a cool gas, passes through a cool gas, dark linesdark lines appear in the continuous spectrum.appear in the continuous spectrum.




Emission and absorption lines Emission and absorption lines explainedexplained

The Bohr model of the atom suggests The Bohr model of the atom suggests that only certain wavelength photons will that only certain wavelength photons will push electrons to higher levels and that push electrons to higher levels and that only certain photons will be emitted only certain photons will be emitted when the electron fall back towards the when the electron fall back towards the ground stateground state

This means that the atom will only emit This means that the atom will only emit and absorb certain wavelengths of light and absorb certain wavelengths of light and will be the at same wavelengthsand will be the at same wavelengths




Color from Reflection - Colors Color from Reflection - Colors of Planetsof Planets

Planets have their colors Planets have their colors because the material on their because the material on their surfaces or in their clouds surfaces or in their clouds absorbs some of the absorbs some of the wavelengths of sunlight and wavelengths of sunlight and reflects a combination of reflects a combination of wavelengths that appear, for wavelengths that appear, for example, as the rusty red of example, as the rusty red of Mars or the blue of Neptune.Mars or the blue of Neptune.




Thermal RadiationThermal Radiation In a low pressure gas atoms will emit at In a low pressure gas atoms will emit at

discrete wavelength (Kirchhoff’s 2discrete wavelength (Kirchhoff’s 2ndnd law) law) If one increases the pressure these If one increases the pressure these

emission will become broader and slight emission will become broader and slight randomized until the emission blur into randomized until the emission blur into each other and form a continuum each other and form a continuum (Kirchhoff’s 1(Kirchhoff’s 1stst law) law)

Any dense object with T > zero K will emit Any dense object with T > zero K will emit a continuum of a continuum of thermal radiationthermal radiation (sometimes called (sometimes called blackbodyblackbody radiation) radiation)



The Colors of Planets and StarsThe Colors of Planets and StarsColor as a Measure of TemperatureColor as a Measure of Temperature An intensity/wavelength graphAn intensity/wavelength graph, , a a thermal thermal

spectrumspectrum,, of an object emitting electromagnetic of an object emitting electromagnetic radiation can be used to determine its radiation can be used to determine its temperature.temperature.

Therefore, the color of a star tells us about its Therefore, the color of a star tells us about its surfacesurface temperature. temperature.

In Mathematical Insight 5.2:In Mathematical Insight 5.2: A A quantitative derivation is given by quantitative derivation is given by Wien’s LawWien’s Law: :

maxmax= 2,900,000/= 2,900,000/T T or or TT = 2,900,000/ = 2,900,000/maxmax

where where TT is the temperature in is the temperature in KelvinKelvin and and m m is is the wavelength where the thermal spectrum the wavelength where the thermal spectrum peaks in intensity in peaks in intensity in nanometers (nm)nanometers (nm)



The Colors of Planets and StarsThe Colors of Planets and Stars

Intensity per square meterIntensity per square meterHow much thermal energy is being How much thermal energy is being

emitted (per square meter) from an emitted (per square meter) from an object with at temperature object with at temperature TT ? ? (see (see Mathematical Insight 5.2)Mathematical Insight 5.2) 4

Energy per square meter TStefan-Boltzmann constant



The Doppler EffectThe Doppler Effect• Doppler effectDoppler effect is the observed change is the observed change

in wavelength from a source moving in wavelength from a source moving toward or away from the observer.toward or away from the observer.

• It is most well known as the change in It is most well known as the change in pitch of sound waves when a speeding pitch of sound waves when a speeding car or train blowing its horn passes car or train blowing its horn passes by.by.

• In front of the moving source one In front of the moving source one hears higher frequency (shorter hears higher frequency (shorter wavelength) sound.wavelength) sound.

• Behind the source one hears lower Behind the source one hears lower frequency (longer wavelength) sound.frequency (longer wavelength) sound.



The Doppler EffectThe Doppler Effect

RedshiftRedshift is the change in wavelength is the change in wavelength toward toward longer wavelengthslonger wavelengths..

BlueshiftBlueshift is the change in wavelength is the change in wavelength toward toward shorter wavelengthsshorter wavelengths..

Except for very distant galaxies, most Except for very distant galaxies, most redshifts or blueshifts caused by the redshifts or blueshifts caused by the Doppler effect are very small.Doppler effect are very small.

It is spectral lines in stellar spectra that It is spectral lines in stellar spectra that make the Doppler effect a powerful tool.make the Doppler effect a powerful tool.



The Doppler EffectThe Doppler EffectDoppler Effect: Measurement TechniqueDoppler Effect: Measurement TechniqueMeasuring the amount of the shifting of stellar Measuring the amount of the shifting of stellar

spectral lines can determine the spectral lines can determine the radial radial velocityvelocity of the star relative to the Earth. of the star relative to the Earth.

vvr

vt

To Earth

Radial velocityRadial velocity is velocity is velocity along the line of sight, toward along the line of sight, toward or away from the observer.or away from the observer.

Tangential velocityTangential velocity is velocity is velocity perpendicular to the line of perpendicular to the line of sight.sight.


0 r

0 0

v

c

0 is the wavelength emitted by the object is the wavelength we observevr is the radial speed of the emitting objectc is the speed of lightNote: A negative v means the distance between the source and observer is decreasing. A positive v means the distance is increasing.

= The difference or shift between the observed wavelength and the wavelength seen if there were no motion






Other Doppler Effect MeasurementsOther Doppler Effect MeasurementsThe rotation rate of the SunThe rotation rate of the SunRotation rates of the planets and the Rotation rates of the planets and the

rings of Saturnrings of SaturnThe motion of other stars with planets The motion of other stars with planets

around themaround themSpeeding cars by police radarSpeeding cars by police radar




Speed measured by the Doppler effect Speed measured by the Doppler effect is the speed of the object is the speed of the object relativerelative to the to the speed of the Earth.speed of the Earth.

All speeds are relative to something. All speeds are relative to something. All motion (or non-motion) is relative, All motion (or non-motion) is relative, too.too.

The understanding of the relativity of The understanding of the relativity of motion is called motion is called Galilean relativityGalilean relativity or or Newtonian relativityNewtonian relativity..



Light IntensityLight Intensity

The energy The energy fluxflux of the wave is the rate at of the wave is the rate at which the wave carries energy through a which the wave carries energy through a given areagiven area

The electromagnetic flux (The electromagnetic flux (FF) decreases ) decreases with square of the distance from the with square of the distance from the source of the waves (source of the waves (inverse square law of inverse square law of radiationradiation):):

24

EF

d

Force of gravity Force of gravity also follows an also follows an inverse square inverse square relationship with relationship with distance.distance.

© sierra college astronomy department 1 light and matter

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