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Week 6 (September 28 & 30) Chapter 5: Light and Matter

Digging deeper into the physical universeDigging deeper into the physical universe

Review some basic pointsReview some basic pointsReview some basic pointsReview some basic points

• Stars “shine” by their own light– EmissionEmission

• Planets, moons, etc. “shine” by reflected light– Reflection

How do we “see”?How do we “see”?How do we “see”?How do we “see”?

• Record Surface Features:– Surface Reflectance

– Thermal Emission

• Digital images transmitted back to Earth via radio signals

LIGHT is the Electromagnetic Spectrum

Fig.5.7

What is light?And so what anyway?

• Light is a form of energy

• We can measure light energy in watts: 1 watt = 1 joule/s

• Visible light is just one special case of all electromagnetic radiation

• Virtually everything we know about objects in space is due to emission (stars or comets) or reflection (planets) of light.

Mass is Energy• Mass is a concentrated form of energy

E = mcE = mc22

•In a Nuclear Fusion reaction: two H atoms are converted to one He atom.•But the mass of the single He is slightly less than the mass of the two H atoms.•The “missing mass” is called the mass defect. This is the m

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Stars Convert Mass to EnergStars Convert Mass to Energ

E = mcE = mc22

Mass is a concentrated form of energy

The Solar Spectrum

Radiation Spectrum of the SunRadiation Spectrum of the Sunmaxmax Sun = 3000 Sun = 3000 K/6000 K = 0.5 K/6000 K = 0.5 m, m, maxmax Earth = 3000 Earth = 3000 K/300 K = 10 K/300 K = 10 mm

•• INPUT: 88% of the sun energy is radiated INPUT: 88% of the sun energy is radiated at wavelength <1.5at wavelength <1.5m m

––44% of the sun’s radiation in the 44% of the sun’s radiation in the visible regionvisible region (0.4(0.4--0.7 0.7 m); 7% as UV; and 37% as near IRm); 7% as UV; and 37% as near IR

Relating color to temperature

Wilhelm Wien

Wien’s Displacement Law

• When you heat an object…– Excite the atoms– Which emit photons

of lightof light– At specific ‘color”

ranges

• The hotter the object, the “bluer” it is.

• The cooler the object, the redder it is.

• Where does nature make things hot?

T is measured in degrees Kelvin

Properties of Radiation

• Hotter objects emit more light at all frequencies per unit area.

• Hotter objects emit photons with a higher average energy.

• Stars are the primary p yproducers of “light” in this universe

• Everything else is just interacting with that light

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Wien’s LawWien’s Law Thought QuestionWhich is hotter?

a) A blue star.

b) A red starb) A red star.

c) A planet that emits only infrared light.

Infrared is real radiation!You just can’t see it with your

naked eye

Your personal IR detector

Don’t Confuse ur IRs

Reflected IR(just beyond visible “red”)

Thermal IR(between VNIR & Microwave)

Visible light is only one ti “ li ”

Colors of “Light” are UniqueColors of “Light” are Unique

• Visible light is made up of many different colors

tiny “slice” of the spectrum

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How do light and matter interact?

• Transmission– Transparent objects transmit light

O bj t bl k ( b b) li ht– Opaque objects block (absorb) light

• Reflection or Scattering

• Emission (AKA radiation)

• Absorption

Light and Matter

Use This Window

Transmission (clear)

Scattering (translucent)

Absorption (opaque)

Today: 9/30

• 52nd anniversary of Sputnik– First object in Earth orbit

– Low-Earth Orbit (LEO)

– About twice the size of a basketball

– Did only one thing

• beep, beep, beep

• Complete Chapter 5:– Making photons

– Spectral analysis

Mt. Pele on the Jovian moon Io

Visible light Reflected Infrared light

The SunToday

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Emission vs Reflection (and absorption)Interactions of Light with Matter

Interactions between light and matter determine the appearance of everything around us

Thought QuestionWhy is a rose red?

a) The rose absorbs blue light.

b) The rose absorbs green light.

c) The rose reflects red light.d) The rose absorbs and then

re-emits red light.

5.2 Properties of Light

• Our goals for learningWh t i li ht?– What is light?• Energy f(wavelength or λ)

• Interactions with matter

What is light?

• A disturbance in the space-time continuum

• A direct conversion of mass to energyMass from an atom– Mass from an atom

– Into energy as a photon

• Light can act either like a wave or like a particle

• Particles of light are called photons

Waves• A wave is a

pattern of motion that can carry energy without carrying matter along with it

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Properties of Waves

• Wavelength is the distance between two wave peaks• Frequency is the number of times per second that a

wave vibrates up and down• Speed of Light (c) = wavelength (λ) x frequency (f)

c = λf or f = c/ λ or λ = c/f

Wavelength and Frequency

wavelength x frequency = speed of light = constant

Electromagnetic WavesTransmit Energy

Higher Frequency → Shorter wavelength → More Energy

Lower Frequency → Longer wavelength → Less Energy

Photons of Light

• Particles of light are called photons

• Each photon has a wavelength and a frequencyfrequency

• The energy of a photon depends on its frequency– REM: Frequency is the number of wave

peaks per second so high frequency means a large number of waves per unit time.

Photons & EnergyPhotons & EnergyThe shorter the wavelength The shorter the wavelength the more energy it carriesthe more energy it carries

(from X(from X--rays and Gamma rays to radio waves)rays and Gamma rays to radio waves)

E = hƒ or hc/E = hƒ or hc/

•• EE is the energy of a is the energy of a h th tphotonphoton

•• ƒƒ is the frequency is the frequency of the corresponding of the corresponding electromagnetic waveelectromagnetic wave

•• is its wavelengthis its wavelength•• hh is Planck’s constant is Planck’s constant

(6.63 10(6.63 10--3434 Joules per Joules per second).second).

Wavelength, Frequency, and Energy

x f = c

= wavelength , f = frequency

c = 3 00 x 108 m/s = speed of lightc = 3.00 x 108 m/s = speed of light

E = h x f = photon energy

h = 6.626 x 10-34 joule x s = photon energy

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Thought Question:Which photon has the most

energy?

a) An x-ray photon.

b) A visible light photon.

c) An infrared photon.

d) A microwave photon

Radiation absorption and emissionRadiation absorption and emissionThe sun radiates energy from fusion at many The sun radiates energy from fusion at many wavelengths. Planets absorb and reflect that wavelengths. Planets absorb and reflect that radiationradiation

Selective absorption Selective absorption in Earth’s in Earth’s AtmosphereAtmosphere

OO22 and Oand O33 absorb almost 100% absorb almost 100% of of the UV the UV at at a a < 0.3 < 0.3 m.m.

0.1 0.3 0.5 0.7 1 5 10 15 20

HH22O and COO and CO22 are strong absorbers of IR radiation are strong absorbers of IR radiation and poor absorbers of visible radiation.and poor absorbers of visible radiation.

transparent

transparent

For For Example…Example…

Water Vapor Image

Red = water vaporGreen = Thermal IRBlue = Reflected IR

CHCH44 and Nand N22O are also O are also strong absorbers of IR radiationstrong absorbers of IR radiation

Common in outer

solar system

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5.3 Properties of Matter

• Our goals for learningWhat is the structure of matter?– What is the structure of matter?

– What are the phases of matter

– How is energy (light) stored in atoms?

ElectronCloud

How Does Matter Make Light?

AtomNucleus

Stay with me. We’re going to find out how light is made!

Atomic Terminology

• Atomic Number = # of protons in nucleus

• Atomic Mass Number = # of protons + neutrons

• Molecules: consist of two or more atoms (H2O, CO2)

Atomic Terminology

• Isotope: same # of protons but different # of neutrons. (4He, 3He)

Phases of matter?

• Familiar phases: – Solid (ice)

– Liquid (water)

– Gas (water vapor)

• Phases of same material behave differently because of differences in chemical bonds

• We’re only doing this so that we can look at the atomic-level analog (and photon emission)

Phases of Water

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Phase Changes

• Ionization: Stripping of electrons, changing atoms into plasma

• Dissociation: Breaking of molecules into atoms

• Evaporation: Breaking of flexible chemical bonds, changing liquid into gas

• Melting: Breaking of rigid chemical bonds, changing solid into liquid

Phases and Pressure

• Phase of a substance depends on both temperature and pressure

• Often more than one phase is present

Meanwhile, at the atomic level…

Excited States

Electrons in atoms are restricted

to particular energy levels

Ground State

Energy Level Transitionsand Photon Generation

• The only allowed changes are those corresponding to

photon release(has a specific energy)

corresponding to exact transitions between energy levels

• Thus the emitted photons will have exact energy levels

• Hence exact wavelengths!

AllowedNot Allowed

Summary

• What is the structure of matter?– Matter is made of atoms, which consist of a nucleus of

protons and neutrons surrounded by a cloud of electrons

• What are the phases of matter? – Adding heat changes phases– As temperature rises, molecules can dissociate atoms– Stripping of electrons from atoms (ionization) turns the

substance into a plasma• Energy Level Transitions

– Create photons with specific energy levels, hence wavelengths

– Wien’s Law makes sense!

5.4 Learning from Light

• Recognizing that “light” is giving us detailed information about what is happening at an atomic, hence compositional, level…

• Our goals for learning– The three basic types of spectra?

– How light tells us what things are made of?

– How light tells us the temperatures of planets and stars?

– How do we interpret an actual spectrum?

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What are the three basic types of spectra?

Continuous Spectrum

Emission Line SpectrumAbsorption Line Spectrum

Spectra of astrophysical objects are usually combinations of these three basic types

Three Types of Spectra Continuous Spectrum

• The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption

Emission Line Spectrum

• A thin or low-density cloud of gas emits light only at specific wavelengths that depend on its composition and temperature, producing a spectrum with bright emission lines

Absorption Line Spectrum

• A cloud of gas between us and a light bulb can absorb light of specific wavelengths, leaving dark absorption lines in the spectrum (also applies to stellar atmospheres)

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How does light tell us what things are made of?

Spectrum of the Sun

Chemical Fingerprints

• Each type of atom has a unique set of energy levels

• Each transition corresponds to a unique photon energy, frequency, and wavelength

Energy levels of Hydrogen

Chemical Fingerprints

• Downward transitions produce a unique pattern of emission lines

Absorption Spectraand Emission Spectrasimultaneously

Chemical Fingerprints

Every element has a unique spectral fingerprint

Test Emission Spectra

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Chemical Fingerprints

• Observing the fingerprints in a spectrum tells us which kinds of atoms are present

Energy Levels of Molecules

• Molecules have additional energy levels because they can vibrate and rotate

Energy Levels of Molecules

Spectrum of Molecular Hydrogen

• The large numbers of vibrational and rotational energy levels can make the spectra of molecules very complicated

• Many of these molecular transitions are in the infrared part of the spectrum

Quick Test:

Which letter(s) labels absorption lines?

A B C D E

Quick Test:Which letter(s) labels the peak

(greatest intensity) of infrared light?

A B C D E

Thought Question

Which letter(s) labels emission lines?

A B C D E

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How do we interpret an actual spectrum?

• By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

What is this object?

Reflected Sunlight: Continuous spectrum of visible light is like the Sun’s except that some of the blue light has been absorbed - object must look red

What is this object?

Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 KMust be pretty cold!

What is this object?

Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the atmosphere

What is this object?

Ultraviolet Emission Lines: Indicate a hot upper atmosphere

What is this object?

Mars!

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Recap• What are the three basic type of spectra?

– Continuous spectrum, emission line spectrum, absorption line spectrum

• How does light tell us what things are• How does light tell us what things are made of?– Each atom has a unique fingerprint.– We can determine which atoms something is

made of by looking for their fingerprints in the spectrum.

And…• How does light tell us the temperatures of

planets and stars?– All stars emit a continuous spectrum that depends on

temperaturetemperature.

– The spectrum of that thermal radiation tells us the object’s temperature.

• How do we interpret an actual spectrum?– By carefully studying the features in a spectrum, we

can learn a great deal about the object that created it.

5.5 The Doppler Effect

• Our goals for learning– How does light tell us the speed of a distant

jobject?

– How does light tell us the rotation rate of an object?

How does light tell us the speed of a distant object?

The Doppler Effect

The Doppler Effect

Same for Light

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Measuring the Shift

Stationary

Moving Away

Moving Away Faster

Measuring the Doppler Effect from shifts in the wavelengths of emission lines

Moving Toward

Moving Toward Faster

The amount of blue or red shift tells us an object’s speed toward or away

ffrom us:

Doppler Shift tells us ONLY about the part of an object’s motion toward or away from us:

Quick Test:

I measure a line in the lab at 500.7 nm.The same line in a star has wavelength 502.8 nm.

450nm=blue 700nm=red

a) It is moving away from me.

b) It is moving toward me.

c) It has unusually long spectral lines.

Redshifted to hereLab Spectra

How does light tell us the rotation rate of an object?

• Different Doppler shifts from different id f t tisides of a rotating

object spread out its spectral lines

Spectrum of a Rotating Object

• Spectral lines are wider when an object rotates faster

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Recap• “Light can tell us:

– What something is made of

– What its temperature is

– If it is a solid or gas

• How does light tell us the speed of a distant object?How does light tell us the speed of a distant object?– The Doppler effect tells us how fast an object is moving toward or away

from us.

• Blueshift: objects moving toward us

• Redshift: objects moving away from us

• How does light tell us the rotation rate of an object?– The width of an object’s spectral lines can tell us how fast it is rotating

End of Chapter 5 slides