chapter 4 radiation and spectra

Astronomy 2010Astronomy 2010 11January 24, 2006January 24, 2006

Chapter 4Chapter 4Radiation and SpectraRadiation and Spectra

The Sunin ultraviolet

January 24, 2006January 24, 2006 Astronomy 2010Astronomy 2010 22

Radiation from Space Radiation from Space Information from the Stars Information from the Stars


The Nature of LightThe Nature of LightAt least 95% of the celestial information we At least 95% of the celestial information we receive is in the form of light. receive is in the form of light.

Astronomers have devised many techniques to Astronomers have devised many techniques to decode as much of the encoded information as decode as much of the encoded information as possible from the small amount of light that possible from the small amount of light that reaches Earth. reaches Earth.

This includes information about the object's This includes information about the object's temperature, motion, chemical composition, temperature, motion, chemical composition, gas density, surface gravity, shape, structure, gas density, surface gravity, shape, structure, and more! and more!


The Nature of Light (cont’d)The Nature of Light (cont’d)Most of the information in light is revealed by Most of the information in light is revealed by using using spectroscopy:spectroscopy:

Spectroscopy is the separation of light into its Spectroscopy is the separation of light into its different constituent colors (or wavelengths) for different constituent colors (or wavelengths) for analysis.analysis.

The resulting components are called the The resulting components are called the spectrumspectrum of the light.of the light.


Electric and Magnetic FieldsElectric and Magnetic FieldsLight is composed of Light is composed of electric electric fieldsfields and and magnetic fieldsmagnetic fields. .

Electric charges and magnets Electric charges and magnets alter the region of space around alter the region of space around them so that they can exert them so that they can exert forces on distant objects. forces on distant objects.

This altered space is called a This altered space is called a force fieldforce field (or just a (or just a fieldfield). ).


ElectromagnetismElectromagnetismConnection between electric and magnetic Connection between electric and magnetic fields was discovered in the 19fields was discovered in the 19thth century. century.

A moving electric charge or an electric current A moving electric charge or an electric current creates a magnetic field. creates a magnetic field.

Coils of wire are used to make the large Coils of wire are used to make the large electromagnets used in car junk yards or the electromagnets used in car junk yards or the tiny electromagnets in your telephone receiver. tiny electromagnets in your telephone receiver.

Electric motors used to start your car or spin a Electric motors used to start your car or spin a computer's hard disk around are other computer's hard disk around are other applications of this phenomenon. applications of this phenomenon.


How it works…How it works…A changing magnetic field creates A changing magnetic field creates electrical current---an electric field.electrical current---an electric field.Concept used in power generators---large Concept used in power generators---large coils of wire are made to turn in a coils of wire are made to turn in a magnetic field magnetic field The coils of wire experience a changing The coils of wire experience a changing magnetic field and electricity is magnetic field and electricity is produced. produced. Computer disks and audio/video tapes Computer disks and audio/video tapes encode information in magnetic patterns... encode information in magnetic patterns... When the magnetic disk or tape material When the magnetic disk or tape material passes by small coils of wire, electrical passes by small coils of wire, electrical currents/fields are produced. currents/fields are produced.


James Clerk MaxwellJames Clerk MaxwellBorn/Educated in ScotlandBorn/Educated in Scotland

Lived 1831—1879Lived 1831—1879

Achieved a synthesis of knowledge Achieved a synthesis of knowledge of electricity and magnetism of his of electricity and magnetism of his time.time.

Hypothesis:Hypothesis:If a changing magnetic field can make If a changing magnetic field can make an electric field, then a changing an electric field, then a changing electric field should make a magnetic electric field should make a magnetic field. field.


Electric and Magnetic Electric and Magnetic FieldsFields

Consequence:Consequence:Changing electric and magnetic fields Changing electric and magnetic fields should trigger each other.should trigger each other.The changing fields move at a speed The changing fields move at a speed equal to the speed of light.equal to the speed of light.

Maxwell’s conclusion.Maxwell’s conclusion.Light Light isis an electromagnetic wave. an electromagnetic wave.

Later experiments confirmed Later experiments confirmed Maxwell's theory.Maxwell's theory.


The electric and magnetic fields oscillate at right angles to each other and the combined wave moves in a direction perpendicular to both of the electric and magnetic field oscillations.


Electromagnetic RadiationElectromagnetic RadiationLight, electricity, and magnetism Light, electricity, and magnetism are manifestations of the same are manifestations of the same thing called thing called electromagnetic electromagnetic radiationradiation..

Electromagnetic radiation is a Electromagnetic radiation is a form of energy.form of energy.

This energy exists in many forms This energy exists in many forms not detectable by our eyes such as not detectable by our eyes such as infrared (IR), radio, X-rays, infrared (IR), radio, X-rays, ultraviolet (UV), and gamma rays. ultraviolet (UV), and gamma rays.


EM Waves General PropertiesEM Waves General PropertiesTravels through Travels through emptyempty space. space. Most other types of waves don’tMost other types of waves don’t

The speed of light (EM radiation) The speed of light (EM radiation) is constant in space. is constant in space. All forms of light have the All forms of light have the samesame speed speed of 299,800 kilometers/second in space of 299,800 kilometers/second in space

This number is abbreviated as This number is abbreviated as cc..

C = fC = f, f = c/, f = c/, , = c/f = c/f


Wave Characteristics (1)Wave Characteristics (1)

-8

-6

-4

-2

0

2

4

6

8

0 5 10 15 20

Time

Wave Amplitude

Period

Amplitude

Amplitude (A)Amplitude (A)A measure of the A measure of the strength/size of the strength/size of the wave.wave.

Period (P)Period (P)Duration of a cycleDuration of a cycleUnits: year, day, Units: year, day, hours, seconds, …hours, seconds, …

Frequency (f)Frequency (f)Rate of repetition of Rate of repetition of a periodic a periodic phenomenon.phenomenon.f=1/Pf=1/PUnits: Hertz (Hz), Units: Hertz (Hz), or cycle/s.or cycle/s.


Period/Frequency ExamplesPeriod/Frequency Examples

PhenomenonPhenomenon PeriodPeriod FrequencyFrequency

Earth’s orbit Earth’s orbit around the Sunaround the Sun 365 days365 days 0.00274 0.00274

daysdays-1-1

Earth’s rotationEarth’s rotation1 day or1 day or

86400 sec86400 sec1 day1 day-1-1 or or

1.16x101.16x10-5-5 Hz Hz

Electrical Power Electrical Power (US)(US) 0.0167 sec0.0167 sec 60 Hz60 Hz

Blue lightBlue light 1.67x101.67x10--

1515 sec sec 6.0x106.0x101414 Hz Hz


Wave Characteristics (2)Wave Characteristics (2)Wavelength (Wavelength ())Size of one cycle of Size of one cycle of the wave in space.the wave in space.Units: meter (m), Units: meter (m), centimeter (cm), centimeter (cm), micrometer (micrometer (m), m), nanometer (nm), nanometer (nm), angstrom (A).angstrom (A).

Velocity (v)Velocity (v)Speed at which the wave Speed at which the wave propagate through propagate through space.space.v = f x v = f x Units: m/s, miles/hour, Units: m/s, miles/hour, km/hour, etc.km/hour, etc.

-8

-6

-4

-2

0

2

4

6

8

0 5 10 15 20

Distance (m)

Wave Amplitude

Wavelength


Distance UnitsDistance Units1 meter = 1 m 1 meter = 1 m

= 100 cm (centimeter)= 100 cm (centimeter)

= 1000 mm (millimeter)= 1000 mm (millimeter)

= 1000000 = 1000000 m (micrometer)m (micrometer)

= 1000000000 nm (nanometer)= 1000000000 nm (nanometer)

= 10000000000 = 10000000000 ÅÅ (Angstrom) (Angstrom)


Visible LightVisible Light

color (Å) f (*1014 Hz) Energy (*10-19 J)

violet 4000 4600 7.5 6.5 5.0 4.3

indigo 4600 4750 6.5 6.3 4.3 4.2

blue 4750 4900 6.3 6.1 4.2 4.1

green 4900 5650 6.1 5.3 4.1 3.5

yellow 5650 5750 5.3 5.2 3.5 3.45

orange 5750 6000 5.2 5.0 3.45 3.3

red 6000 8000 5.0 3.7 3.3 2.5


Color CompositionColor CompositionWhite light is made of different White light is made of different colors (wavelengths). colors (wavelengths).

White light passing through a prism White light passing through a prism or diffraction grating, is spread or diffraction grating, is spread out into its different colors.out into its different colors.

First discovered by Newton


Light Dispersion by Light Dispersion by refractionrefraction

Refraction Angle or dispersion Refraction Angle or dispersion function of the wavelength (color)function of the wavelength (color)

Incidence angle

Refraction angle


Max Planck’s PhotonMax Planck’s PhotonMax PlanckMax Planck (lived 1858--1947) (lived 1858--1947) Discovered that if one considers Discovered that if one considers light light as packets of energy called as packets of energy called photonsphotons, , one can accurately explain the shape one can accurately explain the shape of continuous spectra. of continuous spectra. A A photonphoton is a particle of is a particle of electromagnetic radiation. electromagnetic radiation. Bizarre though it may be, light is Bizarre though it may be, light is both a particle and a wave. both a particle and a wave. Whether light behaves like a wave or Whether light behaves like a wave or like a particle depends on how the like a particle depends on how the light is observedlight is observed

it depends on the experimental setup!it depends on the experimental setup!


Albert Einstein’s Albert Einstein’s Photon Energy Photon Energy Interpretation.Interpretation.

Albert Einstein (lived 1879--1955) A few years after Planck's discovery Albert Einstein found a very simple relationship between the energy of a light wave (photon) and its frequency:Energy of light = h f = (h c)/

where h is a universal constant of nature called ``Planck's constant'' = 6.63 10-34 J·sec.


Characterizing LightCharacterizing LightWe now have three ways to characterize We now have three ways to characterize electromagnetic radiation:electromagnetic radiation:wavelengthwavelength

frequencyfrequency

energyenergy

Astronomers use these interchangeably.Astronomers use these interchangeably.

We also divide the spectrum of all We also divide the spectrum of all possible wavelengths/frequencies/energies possible wavelengths/frequencies/energies into bands that have similar properties. into bands that have similar properties. Light is the most familiar of these.Light is the most familiar of these.


Visible SpectrumVisible Spectrum

Small wavelengthHigh frequencyHigh energy

large wavelengthlow frequencylow energy

Remember the Spectrum: ROY G BIV


The Full SpectrumThe Full SpectrumFrom the highest to lowest energyFrom the highest to lowest energy

Gamma raysGamma rays

X-raysX-rays

Ultraviolet Ultraviolet

VisibleVisible

Infrared Infrared

MicrowaveMicrowave

RadioRadio


EM Waves General Properties EM Waves General Properties (cont’d)(cont’d)

A A wavelengthwavelength of light is defined of light is defined similarly to that of water wavessimilarly to that of water wavesdistance between crests or between distance between crests or between troughs. troughs.

Visible light (what your eye detects) Visible light (what your eye detects) has wavelengths 400-800 nanometers. 1 nm has wavelengths 400-800 nanometers. 1 nm = 10= 10-9-9 m. m.

Radio wavelengths are often measured Radio wavelengths are often measured in centimeters: 1 centimeter = 10in centimeters: 1 centimeter = 10-2-2 meter = 0.01 meter. meter = 0.01 meter. The abbreviation used for wavelength The abbreviation used for wavelength is the Greek letter lambda: is the Greek letter lambda:

The Full E-M SpectrumThe Full E-M Spectrum


EM Radiation Reaching Earth EM Radiation Reaching Earth Not all wavelengths of light from Not all wavelengths of light from space make it to the surface. space make it to the surface. Only long-wave UV, visible, parts of Only long-wave UV, visible, parts of the IR and radio bands make it to the IR and radio bands make it to surface.surface.

More IR reaches elevations above More IR reaches elevations above 9,000 feet (2765 meters) elevation. 9,000 feet (2765 meters) elevation. That is one reason why modern That is one reason why modern observatories are built on top of very observatories are built on top of very high mountains. high mountains.


Earth’s atmosphere is a Earth’s atmosphere is a shieldshield

Blocks gamma rays, X-rays, and most Blocks gamma rays, X-rays, and most UV. UV. Good for the preservation of life on the Good for the preservation of life on the planet…planet…An obstacle for astronomers who study An obstacle for astronomers who study the sky in these bands. the sky in these bands.

Blocks most of the IR and parts of Blocks most of the IR and parts of the radio. the radio. Astronomers unable to detect these forms Astronomers unable to detect these forms of energy from celestial objects from of energy from celestial objects from the groundthe groundMust resort to very expensive satellite Must resort to very expensive satellite observatories in orbit. observatories in orbit.


Types of SpectraTypes of SpectraContinuous spectraContinuous spectra consist of all consist of all frequencies (colors) - the thermal frequencies (colors) - the thermal or blackbody spectrum is the most or blackbody spectrum is the most common example we will see.common example we will see.Absorption line spectra Absorption line spectra are are continuous spectra with certain continuous spectra with certain missing certain frequencies.missing certain frequencies.Emission line spectraEmission line spectra are a series are a series of discrete frequencies (with or of discrete frequencies (with or without a continuous spectra).without a continuous spectra).Often astronomers deal with Often astronomers deal with combinations of the above.combinations of the above.


Black Body SpectrumBlack Body Spectrum

Star Color Star Color vs. vs.

TemperatureTemperature


Discrete SpectraDiscrete SpectraClose examination of the spectra from the Sun Close examination of the spectra from the Sun and other stars reveals that the rainbow of and other stars reveals that the rainbow of colors has many dark lines in it, called colors has many dark lines in it, called absorption linesabsorption lines. .

They are produced by the cooler thin gas in the They are produced by the cooler thin gas in the upper layers of the stars absorbing certain upper layers of the stars absorbing certain colors of light produced by the hotter dense colors of light produced by the hotter dense lower layers. lower layers.

The spectra of hot, thin (low density) gas The spectra of hot, thin (low density) gas clouds are a series of bright lines called clouds are a series of bright lines called emission linesemission lines. .

In both of these types of spectra you see In both of these types of spectra you see spectral features at certain, discrete spectral features at certain, discrete wavelengths (or colors) and no where else. wavelengths (or colors) and no where else.

Absorption Line SpectrumAbsorption Line Spectrum


SpectraSpectraThe type of spectrum you see depends The type of spectrum you see depends on the temperature of the thin gas. on the temperature of the thin gas. If the thin gas is If the thin gas is coolercooler than the thermal than the thermal source in the background, you see source in the background, you see absorption lines. absorption lines. Since the spectra of stars show absorption Since the spectra of stars show absorption lines, it tells you that the density and lines, it tells you that the density and temperature of the upper layers of a star temperature of the upper layers of a star is lower than the deeper layers. is lower than the deeper layers. In a few cases you can see emission lines In a few cases you can see emission lines on top of the thermal spectrum. This is on top of the thermal spectrum. This is produced by thin gas that is produced by thin gas that is hotterhotter than than the thermal source in the background. the thermal source in the background.


Spectra (cont’d)Spectra (cont’d)The spectrum of a hydrogen-The spectrum of a hydrogen-emission nebula (``nebula'' = gas emission nebula (``nebula'' = gas or dust cloud) is just a series of or dust cloud) is just a series of emission lines without any thermal emission lines without any thermal spectrum because there are no stars spectrum because there are no stars visible behind the hot nebula. visible behind the hot nebula.

Some objects produce spectra that Some objects produce spectra that are a combination of a thermal are a combination of a thermal spectrum, emission lines, and spectrum, emission lines, and absorption lines simultaneously!absorption lines simultaneously!


Spectra (cont’d)Spectra (cont’d)


The Structure of the AtomThe Structure of the Atom


Bohr atomBohr atom

Explanation for the Explanation for the discrete line spectradiscrete line spectra … …

Niels BohrNiels Bohr (lived 1885--1962) provided the (lived 1885--1962) provided the explanation in the early 20th century. explanation in the early 20th century.

Electrons only exist in certain Electrons only exist in certain energy levelsenergy levels and as and as long as an electron stays in a particular energy long as an electron stays in a particular energy level, it doesn’t emit any energy (level, it doesn’t emit any energy (photonsphotons).).

If an electron changes energy levels, it emits or If an electron changes energy levels, it emits or absorbs energy in the form of a photon.absorbs energy in the form of a photon.

Set of energies (light frequencies) uniquely identify Set of energies (light frequencies) uniquely identify the type of atom! the type of atom!


Bohr’s ModelBohr’s ModelIn Bohr's model of the atom, the massive but In Bohr's model of the atom, the massive but small positively-charged protons and massive but small positively-charged protons and massive but small neutral neutrons are found in the tiny small neutral neutrons are found in the tiny nucleus. nucleus. The small, light negatively-charged electrons The small, light negatively-charged electrons move around the nucleus in certain specific move around the nucleus in certain specific orbits (energy levels). orbits (energy levels). In a neutral atom the number of electrons = the In a neutral atom the number of electrons = the number of protons. number of protons. The arrangement of an atom's energy levels The arrangement of an atom's energy levels (orbits) depends on the number of protons (and (orbits) depends on the number of protons (and neutrons) in the nucleus and the number of neutrons) in the nucleus and the number of electrons orbiting the nucleus. electrons orbiting the nucleus. Because every type of atom has a unique Because every type of atom has a unique arrangement of energy levels, they produce a arrangement of energy levels, they produce a unique pattern of absorption or emission lines.unique pattern of absorption or emission lines.


IsotopesIsotopes


How is light produced?How is light produced?

Absorption Line SpectraAbsorption Line Spectra


Doppler EffectDoppler EffectThe wave nature of light means there will be a The wave nature of light means there will be a shift in the spectral lines of an object if it shift in the spectral lines of an object if it is moving.is moving.

Sound Waves: pitch Sound Waves: pitch frequency of wave frequency of wave

Changes the pitch of the sound coming from Changes the pitch of the sound coming from something moving toward you or away from you something moving toward you or away from you

train whistle, police siren train whistle, police siren

Sounds from objects moving toward you are at a Sounds from objects moving toward you are at a higher pitch because the sound waves are higher pitch because the sound waves are compressed together, shortening the wavelength compressed together, shortening the wavelength of the sound waves. of the sound waves.

Sounds from objects moving away from you are Sounds from objects moving away from you are at a lower pitch because the sound waves are at a lower pitch because the sound waves are stretched apart, lengthening the wavelength. stretched apart, lengthening the wavelength.


Sound WavesSound Waves

•Spread uniformly from a sound source•Circles -- crests of the sound waves•Think of waves spreading from a pebble dropped into a pool


Doppler ShiftDoppler Shift

Moving towards

Short wavelength

Moving away

longer wavelength


Doppler Shift Doppler Shift Speed of Speed of SourceSource


Red shift, Blue shiftRed shift, Blue shiftMotion of the light source causes the spectral Motion of the light source causes the spectral lines to shift positions.lines to shift positions.Which way the spectral lines shift tells you if Which way the spectral lines shift tells you if the object is moving toward or away from you. the object is moving toward or away from you.

Blue shift:Blue shift: If the object is moving toward you, the If the object is moving toward you, the waves are compressed, so their wavelength is waves are compressed, so their wavelength is shorter. The lines are shifted to shorter (bluer) shorter. The lines are shifted to shorter (bluer) wavelengths. wavelengths. Red shift:Red shift: If the object is moving away from you, If the object is moving away from you, the waves are stretched out, so their wavelength is the waves are stretched out, so their wavelength is longer. The lines are shifted to longer (redder) longer. The lines are shifted to longer (redder) wavelengths.wavelengths.

The doppler effect will not affect the overall The doppler effect will not affect the overall color of an object unless it is moving at a color of an object unless it is moving at a significant fraction of the speed of light significant fraction of the speed of light (VERY fast!) (VERY fast!)


Doppler Shifted SpectraDoppler Shifted Spectra


Doppler Shift (5)Doppler Shift (5)


Doppler Shift (6)Doppler Shift (6)


Expanding UniverseExpanding Universe

The doppler effect tells us about The doppler effect tells us about the relative motion of stars with the relative motion of stars with respect to us. respect to us.

The spectral lines of nearly all of The spectral lines of nearly all of the galaxies in the universe are the galaxies in the universe are shifted to the red end of the shifted to the red end of the spectrum. spectrum.

These galaxies are moving away from These galaxies are moving away from our Milky Way galaxy. our Milky Way galaxy.

This is evidence for the expansion This is evidence for the expansion of the universe.of the universe.

chapter 4 radiation and spectra

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