# Chapter 5: Light: The Cosmic Messenger. What is Light? Light is radiative energy Energy is measured in Joules Power is measured in Watts 1 watt = 1 joule/s.

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Chapter 5: Light: The Cosmic Messenger

What is Light? Light is radiative energy Energy is measured in Joules Power is measured in Watts 1 watt = 1 joule/s The power a person uses in a day is about 10 Mjoules, equivalent to leaving a 100W bulb on all day

How Light Behaves EmissionAbsorptionTransmissionReflection

If you pass white light through a prism, it separates into its component colors. R.O.Y. G. B.I.V spectrum long wavelengthsshort wavelengths

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

© 2014 Pearson Education, Inc.Waves A wave is a pattern of motion that can carry energy without carrying matter along with it.

© 2014 Pearson Education, Inc. 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. Wave speed = wavelength x frequency

© 2014 Pearson Education, Inc. Light: Electromagnetic Waves A light wave is a vibration of electric and magnetic fields. Light interacts with charged particles through these electric and magnetic fields.

Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a combination of electric and magnetic fields.

© 2014 Pearson Education, Inc. Wavelength and Frequency wavelength x frequency = speed of light = constant

© 2014 Pearson Education, Inc. 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 frequency.

Thomas Young’s interference experiment

The Electromagnetic Spectrum

Light 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 wavelengths 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 objectcooler object

The intensities of different emitted colors reveal a star’s temperature Wien’s Law Wavelength is inversely proportional to temperature max = (2.9 x 10 -3 ) / T Kelvin max = (2.9 x 10 -3 ) / T Kelvin As Energy goes up, Temperature increases and wavelength gets smaller, and frequency gets greater

What color is our 5800K Sun? The Sun emits all colors (wavelengths of electromagnetic radiation); however, the colors it emits most intensely are in the blue-green part of the spectrum.

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

© 2014 Pearson Education, Inc. Chemical Fingerprints Each type of atom has a unique set of energy levels. Each transition corresponds to a unique photon energy, frequency, and wavelength.

© 2014 Pearson Education, Inc. Chemical Fingerprints Each type of atom has a unique spectral fingerprint.

© 2014 Pearson Education, Inc. Chemical Fingerprints Observing the fingerprints in a spectrum tells us which kinds of atoms are present.

The Sun’s Spectrum

The brightness of spectral lines depend on conditions in the spectrum’s source Law 1 A hot object or a hot, dense gas produces a continuous spectrum -- a complete rainbow of colors with without any specific spectral lines. (This is a black body spectrum.) Law 2 A hot, rarefied gas produces an emission line spectrum - a series of bright spectral lines against a dark background. Law 3 A cool gas in front of a continuous source of light produces an absorption line spectrum - a series of dark spectral lines among the colors of the rainbow.

Absorption Spectrum of Hydrogen Gas

Kirchhoff’s Laws

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

Doppler Shifts Red Shift: The distance between the observer and the source is increasing Blue Shift: The distance between the observer and the source is decreasing

The Doppler shift allows astronomers to measure radial velocity ONLY! The proper motion, side-to-side, is too difficult to measure for all but the closest stars (that would require we can measure via parallax). With Doppler Shift we can measure the speed of objects toward or away from us very precisely!

A star’s surface temperature –by peak wavelength A star’s chemical composition –by spectral analysis A star’s radial velocity –from Doppler shifts

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