1. Light: The Cosmic Messenger
Chapter 5
Light: The Cosmic Messenger
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2. What is light?
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3. Light is an electromagnetic wave.
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4. Wavelength and Frequency
wavelength  frequency = speed of light = constant
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6. Reflection and Scattering
Movie screen scatters light in all directions.
Mirror reflects light in a particular direction.
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7. Interactions of Light with Matter
Interactions between light and matter determine the appearance of everything around us.
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8. Thought Question Why is a rose red? • The rose absorbs red light. •
The rose transmits red light. •
The rose emits red light. •
The rose reflects red light.
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9. Thought Question Why is a rose red? • The rose absorbs red light. •
The rose transmits red light. •
The rose emits red light.
The rose reflects red light.
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10. Continuous Spectrum •
The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.
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11. 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.
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12. 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.
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13. How does light tell us what things are made of?
Spectrum of the Sun
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14. Chemical Fingerprints•
Each type of atom has a unique spectral fingerprint.
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15. Chemical Fingerprints•
Observing the fingerprints in a spectrum tells us which kinds of atoms are present.
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16. How does light tell us the temperatures
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17. Thermal Radiation •
Nearly all large or dense objects emit thermal radiation, including stars, planets, and you. •
An object’s thermal radiation spectrum depends on only one property: its temperature.
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18. Properties of Thermal
Hotter objects emit more light at all frequencies per
unit area.
Hotter objects emit photons with a higher average
energy.
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19. Thought Question Which is hotter? • A blue star • A red star •
A planet that emits only infrared light
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20. Thought Question Which is hotter? A blue star • A red star •
A planet that emits only infrared light
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21. Thought Question Why don’t we glow in the dark? •
People do not emit any kind of light.
People only emit light that is invisible to our eyes.
People are too small to emit enough light for us to see.
People do not contain enough radioactive material. • •
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22. Thought Question Why don’t we glow in the dark?
People do not emit any kind of light.
People only emit light that is invisible to our eyes.
People are too small to emit enough light for us to see.
People do not contain enough radioactive material. • •
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23. Interpreting an Actual Spectrum•
By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.
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24. 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
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25. What is this object?
Thermal Radiation: Infrared spectrum peaks at a wavelength corresponding to a temperature of 225 K
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26. Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the
What is this object?
Carbon Dioxide: Absorption lines are the fingerprint of CO2 in the
atmosphere
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27. Ultraviolet Emission Lines: Indicate a hot upper atmosphere
What is this object?
Ultraviolet Emission Lines: Indicate a hot upper atmosphere
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28. What is this object?
Mars!
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29. How does light tell us the speed of a distant object?
The Doppler Effect
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30. Same for light:
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32. Thursday, February 2, 12

33. Measuring the Shift
Stationary
Moving Away
Away Faster Moving Toward
Toward Faster •
We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.
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34. What can I say about this star?
Thought Question
I measure a line in the lab at nm. The same line in a star has wavelength nm.
What can I say about this star? •
It is moving away from me. •
It is moving toward me. •
It has unusually long spectral lines.
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35. What can I say about this star?
Thought Question
I measure a line in the lab at nm. The same line in a star has wavelength nm.
What can I say about this star?
It is moving away from me. •
It is moving toward me. •
It has unusually long spectral lines.
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36. Measuring Velocity from Redshift:
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37. Example: One of the visible lines of hydrogen has a rest wavelength of nm, but it appears in the spectrum of the star Vega at nm. How is Vega moving relative to us?
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38. Solution: We use the rest wavelength and the shifted wavelength
= nm
nm:
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40. Vega is moving toward us at 13.7 km per second
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41. Thursday, February 2, 12