Chapter 5:Chapter 5:

Light: The Cosmic Light: The Cosmic MessengerMessenger

What is Light?What is Light?

Light is radiative energyLight is radiative energy

Energy is measured in JoulesEnergy is measured in Joules

Power is measured in WattsPower is measured in Watts

1 watt = 1 joule/s1 watt = 1 joule/s

The power a person uses in a day is about The power a person uses in a day is about 10 Mjoules, equivalent to leaving a 100W 10 Mjoules, equivalent to leaving a 100W bulb on all daybulb on all day

How Light BehavesHow Light Behaves

EmissionEmission

AbsorptionAbsorption

TransmissionTransmission

ReflectionReflection

If you pass white light through a prism, If you pass white light through a prism, it separates into its component colors.it separates into its component colors.

R.O.Y. G. B.I.V

spectrum

long wavelengths short wavelengths

Duality of LightDuality of Light

Light can behave as a particle and a waveLight can behave as a particle and a waveIn the 17th Century, Isaac Newton argued that In the 17th Century, Isaac Newton argued that light was composed of little particles while light was composed of little particles while Christian Huygens suggested that light travels in Christian Huygens suggested that light travels in the form of waves.the form of waves.In the 19th Century, Thomas Young In the 19th Century, Thomas Young demonstrated that light bends slightly around demonstrated that light bends slightly around corners and acts like interfering waves.corners and acts like interfering waves.

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WavesWaves

A A wavewave is a is a pattern of motion pattern of motion that can carry that can carry energy without energy without carrying matter carrying matter along with it.along with it.

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Properties of WavesProperties of Waves

WavelengthWavelength is the distance between two wave peaks. is the distance between two wave peaks.FrequencyFrequency is the number of times per second that a is the number of times per second that a wave vibrates up and down.wave vibrates up and down.

Wave speed = wavelength x frequencyWave speed = wavelength x frequency

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Light: Electromagnetic Waves Light: Electromagnetic Waves

A light wave is a vibration of electric and magnetic fields.A light wave is a vibration of electric and magnetic fields.Light interacts with charged particles through these Light interacts with charged particles through these electric and magnetic fields.electric and magnetic fields.

Scottish physicist James Clerk Maxwell showed Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a mathematically in the 1860s that light must be a

combination of electric and magnetic fields.combination of electric and magnetic fields.

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Wavelength and FrequencyWavelength and Frequency

wavelength x frequency = speed of light = constantwavelength x frequency = speed of light = constant

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Particles of LightParticles of Light

Particles of light are called Particles of light are called photons.photons.

Each photon has a wavelength and a frequency.Each photon has a wavelength and a frequency.

The energy of a photon depends on its The energy of a photon depends on its frequency.frequency.

The Electromagnetic SpectrumThe Electromagnetic Spectrum

Light and EnergyLight and Energy

Photon energy = Plank’s constant x speed of light / wavelength

E = h x f = hc/λ

As Energy goes up, frequency goes up, wavelength gets smaller

Peak color (wavelength) shifts to shorter Peak color (wavelength) shifts to shorter wavelengths as an objects is heatedwavelengths as an objects is heated

increasing temperature

Peak color (wavelength) emitted depends on an object’s

temperature

Peak color (wavelength) shifts to shorter wavelengths as an objects is heated

hotter object cooler object

The intensities of different emitted The intensities of different emitted colors reveal a starcolors reveal a star’’s temperatures temperature

WienWien’’s Laws LawWavelength is inversely proportional to temperatureWavelength is inversely proportional to temperature

maxmax = (2.9 x 10 = (2.9 x 10-3-3) ) // T TKelvinKelvin

As Energy goes up, Temperature increases and As Energy goes up, Temperature increases and wavelength gets smaller, and frequency gets greaterwavelength gets smaller, and frequency gets greater

Each chemical element produces its own unique set of spectral lines when it burns

Spectral lines occur when an electron jumps from one energy level to another

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Chemical Fingerprints• Each type of

atom has a unique set of energy levels.

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

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Chemical Fingerprints

• Each type of atom has a unique spectral fingerprint.

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Chemical Fingerprints

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

The SunThe Sun’’s Spectrums Spectrum

The brightness of spectral lines depend on The brightness of spectral lines depend on conditions in the spectrumconditions in the spectrum’’s sources source

Law 1Law 1 A hot object or a hot, dense gas produces a A hot object or a hot, dense gas produces a continuous spectrumcontinuous spectrum -- a complete rainbow of colors -- a complete rainbow of colors with without any specific spectral lines. with without any specific spectral lines. (This is a black (This is a black body spectrum.)body spectrum.)

Law 2Law 2 A hot, rarefied gas produces an A hot, rarefied gas produces an emission line emission line spectrumspectrum - a series of bright spectral lines against a - a series of bright spectral lines against a dark background.dark background.

Law 3Law 3 A cool gas in front of a continuous source of light A cool gas in front of a continuous source of light produces an produces an absorption line spectrumabsorption line spectrum - a series of - a series of dark spectral lines among the colors of the rainbow.dark spectral lines among the colors of the rainbow.

Absorption Spectrum of Hydrogen Gas

KirchhoffKirchhoff’’s Lawss Laws

We can determine a starWe can determine a star’’s movement by s movement by observing the shift in spectrum of the light observing the shift in spectrum of the light

from a starfrom a star

BarnardBarnard’’s Stars Star

Doppler ShiftsDoppler Shifts

Red ShiftRed Shift: The distance between the : The distance between the observer and the source is observer and the source is increasingincreasing

Blue ShiftBlue Shift: The distance between the : The distance between the observer and the source is observer and the source is decreasingdecreasing

The Doppler shift The Doppler shift allows astronomers allows astronomers

to measureto measureradial velocityradial velocity

ONLY!ONLY! The proper motion, side-to-side,is too difficult to measurefor all but the closeststars (that would requirewe can measure via parallax).With Doppler Shiftwe can measure the speed ofobjects toward or away from usvery precisely!

A starA star’’s s surface temperature surface temperature – by peak wavelengthby peak wavelength

A starA star’’s s chemical composition chemical composition – by spectral analysisby spectral analysis

A starA star’’s s radialradial velocity velocity – from Doppler shiftsfrom Doppler shifts