Chapter 5 Light And Matter: Reading Messages From The

Chapter 5Light and Matter:Reading Messages from the Cosmos

Messages Interactions of Light and Matter– The interactions determine everything we see,including what we observe in the Universe. What is light?– Wave AND Particle Electromagnetic spectrum– Separate light into color (in detail)– Tell a lot about the object

5.1 Light in Everyday LifeOur goals for learning: How do we experience light? How do light and matter interact?

How do we experience light? The warmth of sunlight tells us that light isa form of energy We can measure the flow of energy in lightin units of watts: 1 watt 1 joule/s

How do light and matter interact? Emission Absorption Transmission– Transparent objects transmit light– Opaque objects block (absorb) light Reflection or Scattering

Colors of LightPrism White light is made up of many different colorsSeparating the colors gives you important information of anobject (we will learn this later in this Chapter).

Colors in Sun LightRain drops work as prism.Different colors of light arerefracted in different angles andare separated.

Interactions of Light with MatterReflection and ScatteringMirror reflectslight in a particulardirectionMovie screen scatters lightin all directionsThe surface of screen is not flat, soreflects lights in all directions.

Interactions of Light with MatterSun light (white light) has all colors and shines a rose with allthe colors. But the rose looks red, not white. – why?Because the red rose reflects only red light.Hence, from the red color, we can learn that the rose is made of amaterial that reflects only red light.à Astronomers use colors to learn about astronomical objects.

Interactions of Light with MatterWhich cloth is warmer in sun light?Black cloths – because black absorbs more lights, and that’swhy it looks black (no color).Interactions between light and matter determine theappearance of everything around us

Interactions of Light with MatterOnly green light can be transmitted (pass through) this material.Interactions between light and matter determine theappearance of everything around us

What have we learned? How do we experience light?– Light is a form of energy– Light comes in many colors that combine toform white light. How does light interact with matter?– Matter can emit light, absorb light, transmitlight, and reflect (or scatter) light.– Interactions between light and matterdetermine the appearance of everything wesee.

5.2 Properties of LightOur goals for learning: What is light? What is the electromagnetic spectrum?Light Wave & Particle

What is light? Light can act either like a wave or like a particle Particles of light are called photonsLight is very strange. It carries energybut has no mass.

Waves A wave is apattern ofmotion that cancarry energywithout carryingmatter alongwith it

Properties of WavesTwo important quantities of waves Wavelength is the distance between two wave peaks Frequency is the number of times per second that awave vibrates up and downwavelength x frequency wave speed

Light: Electromagnetic Waves A light wave is a vibration of electric and magnetic fields Light interacts with charged particles through theseelectric and magnetic fields

Wavelength and Frequencywavelength x frequency speed of light constant

Particles of Light Particles of light are called photons Each photon has a wavelength and a frequency The energy of a photon depends on its frequencyDigital camera counts the number of photons.

Wavelength, Frequency, and EnergyWavelength and Frequencyλx f cλ wavelength , f frequencyc 3.00 x 108 m/s speed of lightEnergyE h x f photon energyh 6.626 x 10-34 joule x s photon energyColor difference in wavelength.Red light: longer wavelength lower frequency lower energyBlue light: shorter wavelength higher frequency higher energy

What is the electromagnetic spectrum?

Electromagnetic SpectrumColor is a part of an electromagnetic spectrum that your eyecan see.

Electromagnetic SpectrumBlue sidewavelength shorterRed sidelongerfrequencyhigherlowerAtomFootball Fieldwavelength x frequency speed of light constant

Different wavelengths showdifferent thingswavelength shorterBlue sidelongerRed sideRemember: Light carries energy.Which side of the electromagnetic spectrum carries more energy?Energyhigherlower

Electromagnetic SpectrumDifferent wavelengths show different things – different kindsof astronomical objects and their properties

Messages Three types of spectrum What can we learn from object’s light?

Graph of Electromagnetic SpectrumIntensity strength of light at the wavelengthWavelength

What have we learned? What is light?– Light can behave like either a wave or aparticle– A light wave is a vibration of electric andmagnetic fields– Light waves have a wavelength and afrequency– Photons are particles of light. What is the electromagnetic spectrum?– Human eyes cannot see most forms of light.– The entire range of wavelengths of light isknown as the electromagnetic spectrum.

5.3 Properties of MatterOur goals for learning: What is the structure of matter? What are the phases of matter? How is energy stored in atoms?

What is the structure of matter?What do they consist of?

What is the structure of matter?Cut matter (e.g., tofu, but any matter) to half, then half, and half, half,half . How small can we get?

Structure of AtomCutting any material half, half, à Eventually you will find atoms.ElectronVery small particleAtomNucleusProtonNeutronAtom is made out of nucleus and electrons. Nucleus turned out tobe made out of even smaller particles (proton and neutron).

Many kinds of atom

Atomic Terminology Atomic Number # of protons in nucleus Atomic Mass Number # of protons neutrons Molecules: consist of two or more atoms (H2O, CO2)

Electron Orbits in AtomElectrons are orbiting around the nucleus.

Fraunhofer LinesLight and electron orbits in atoms turned out to be closelyrelated.If we look at Sun’s rainbow VERY (and VERY!) closely, we findsome parts are black (no/little light).

Electron Orbits in AtomElectron orbits have strangeshapes, but this is too advanced,and let’s forget about it now.IMPORTANT: Each atom has its own set of electron orbits. The orbits are fixed. Each orbit has a fixed energy.

How is energy stored in atoms?EnergyhighExcited StateslowGround State Electrons in atoms are restricted to particularenergy levels (orbits)Note: this is a story inside an atom.

Energy Level Transitions The only allowedchanges inenergy are thosecorresponding toa transitionbetween energylevelsNot AllowedAllowedSometimes, electrons jump from one energy level to the other.But wait. Where does the energy go? The energy should be conserved

Energy Level Transitionsà Absorption or Emission of Photon!YES. The energy must be conserved.Absorb a photonEmit a photonUpward jumpElectron getsthe energyDownward jumpElectron looses theenergyIn this example, the energy of a photon is exactly the sameas the energy difference between level 1 and level 2.The energy difference btw two levels photon energy à wavelength

Line spectrumEmission Line SpectrumAbsorption Line SpectrumElectrons occasionally jump from one energy level to the other.à Emit or absorb photons at particular energies (wavelengths).

Chemical Fingerprints Each type of atomhas a unique set ofenergy levels Each transitioncorresponds to aunique photonenergy, frequency,and wavelengthEnergy levels of Hydrogen

Chemical Fingerprints Each type of atom has a unique spectral fingerprint

Chemical Fingerprints Observing the fingerprints in a spectrum tells uswhich kinds of atoms are present

AuroraGreen: Oxygen line emissionsRed: Nitrogen line emissionsAurora from Space Shuttle

Example spectrum of the SunEach atom has a particular setof energy levelsà emit photons with particularenergies (wavelengths).à from the pattern of lineemissions in spectrum, we canidentify what atoms are there.wavelengthThe dark pattern (absorption line spectrum) tells us what atoms arethere. We can compare sun’s spectrum with the spectrum of atomsmeasured in laboratory and find what atoms are in the Sun.

What have we learned? What is the structure of matter?– Matter is made of atoms, which consist of anucleus of protons and neutrons surrounded bya cloud of electrons What are the phases of matter?– Adding heat to a substance changes its phase bybreaking chemical bonds.– As temperature rises, a substance transformsfrom a solid to a liquid to a gas, then themolecules can dissociate into atoms– Stripping of electrons from atoms (ionization)turns the substance into a plasma

What have we learned? How is energy stored in atoms?– The energies of electrons in atoms correspondto particular energy levels.– Atoms gain and lose energy only in amountcorresponding to particular changes in energylevels.

5.4 Learning from LightOur goals for learning: What are the three basic types of spectra? How does light tell us what things are madeof? How does light tell us the temperatures ofplanets and stars? How do we interpret an actual spectrum?

What are the three basic types ofspectra?Continuous SpectrumEmission Line SpectrumAbsorption Line SpectrumSpectra of astrophysical objects are usually combinations ofthese three basic types

Emission Line Spectrum A thin or low-density cloud of gas emits light onlyat specific wavelengths that depend on itscomposition and temperature, producing aspectrum with bright emission lines

Absorption Line Spectrum A cloud of gas between us and a lightbulb can absorb light of specificwavelengths, leaving dark absorptionlines in the spectrumNeed photon to absorb

Continuous Spectrum The spectrum of a common (incandescent) lightbulb spans all visible wavelengths, withoutinterruptionAll wavelengths are completely filled.

How does light tell us whatthings are made of?Spectrum of the Sun

Chemical Fingerprints Because thoseatoms can absorbphotons with thosesame energies,upward transitionsproduce a patternof absorption linesat the samewavelengths

Energy Levels of Molecules Molecules have additional energy levels becausethey can vibrate and rotate -- different patterns ofline emission and absorption

Energy Levels of Molecules The large numbers of vibrational and rotationalenergy levels can make the spectra of molecules verycomplicated Many of these molecular transitions are in theinfrared part of the spectrum

How does light tell us thetemperatures of planets and stars?

Thermal RadiationIron glows (emits visible light) when it is hot.

Thermal RadiationOptical lightInfrared lightYour body emits light, too, but you cannot see it with your eyebecause your light is in the range of infrared wavelength (outside therange that your eye can see).

Thermal Radiation Nearly all large or dense objects emit thermalradiation, including stars, planets, you An object’s thermal radiation spectrum dependson only one property: its temperature Themral radiation spectrum is a continuousspectrum.

Properties of Thermal RadiationHotter objects1. emit more light at all frequencies per unit area.2. emit photons with a higher average energy.3. are bluer (whiter).BluerRedder

Color of Stars

Color of StarsColor of stars depends on their temperature.More massive stars have higher temperature, therefore arebluer (whiter) than less massive stars (Chapter 15)

How do we interpret an actualspectrum? By carefully studying the features in aspectrum, we can learn a great deal aboutthe object that created it.

What have we learned? What are the three basic type of spectra?– Continuous spectrum– Emission line spectrum– Absorption line spectrum How does light tell us what things aremade of?– Each atom has a unique fingerprint.– We can determine which atoms something ismade of by looking for their fingerprints inthe spectrum.