Chapter 5 Light: The Cosmic Messenger
5.1 Basic Properties of Light and Matter Our goals for learning: • What is light? • What is matter? • How do light and matter interact?
What is light?
Light is an electromagnetic wave.
Anatomy of a Wave
Wavelength and Frequency wavelength × frequency = speed of light = constant
The Electromagnetic Spectrum Electromagnetic Spectrum
No photons • The book talks about photons • Looks this way because of matter
Which type of electromagnetic radiation has the longest wave- length? 1. Ultraviolet 2. Visible 3. X-ray 4. Infrared 5. Radio
violet colored light has a wavelength of 400 nm and red has a wavelength of 700 nm. How do their frequencies compare? How do their speeds compare? 1. Violet light is faster 2. Red light is faster 3. They have the same speed 4. You can not compare them
Radio waves have a frequency of about 100 Mhz. X-rays have a frequency of about 10^17 Hz. Which is faster? 1. Radio waves are faster 2. X-rays are faster 3. They have the same speed 4. You can not compare them
Atoms • Greeks - indivisible
Dalton (1800) • Every element is made of atoms • All atoms are the same size • Different elements = different atoms • Compound = combo of elements
Which of Dalton’s ideas below do we no longer believe? 1. Elements contain only one type of atom 2. Atoms rearrange in chemical reactions 3. Atoms are solid masses that can’t be split into smaller particles
Which of the following are elements? 1. Plastic 2. Oxygen 3. Carbon 4. Gold 5. Water 6. 1, 2, 3, 4 7. 2, 3, 4, 5 8. 1, 3, 4 9. 2, 3, 4 10. All of the above
J. Thomson (~1900) • Electron • Smaller than an atom
Which statement about Thomson’s model of the atom is true? 1. The charge on the electrons is far greater than the positive charge in the atom. 2. The total charge carried by the electrons equals the positive charge in the atom.
Rutherford ~1910 • Alpha particles shot at gold foil
In which two ways did Rutherford change Thomson’s model of the atom? 1. He said the electrons were concentrated in the center of the atom (nucleus) 2. He said that the positive charge was concentrated in the center of the atom (nucleus) 3. He said that the electrons were orbiting the center of the atom (nucleus) 4. 1 and 2 5. 1 and 3 6. 2 and 3
Niels Bohr (1914) • Light from atoms had specific frequencies • Bohr model has electrons at certain energies
How did Bohr change Rutherford’s model of the atom? 1. Electrons were at the center of the atom 2. Electrons could only occupy specific energy levels 3. Electrons could orbit in a complete random way
Schrodinger and Heisenburg (~1920)
Who proposed the “plum pudding” model of the atom? 1. Dalton 2. Thomson 3. Rutherford 4. Bohr
Which statement is correct? 1. Most of the volume of an atom is taken up by a large nucleus 2. Atoms are mostly space
What is matter?
Atomic Terminology • Atomic Number = # of protons in nucleus • Atomic Mass Number = # of protons + neutrons
Atomic Terminology • Isotope: same # of protons but different # of neutrons ( 4 He, 3 He) • Molecules: consist of two or more atoms (H 2 O, CO 2 )
How do light and matter interact? • Emission • Absorption • Transmission: — Transparent objects transmit light. — Opaque objects block (absorb) light. • Reflection or scattering
Reflection and Scattering Mirror reflects Movie screen scatters light light in a in all directions. particular direction.
How do we see
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. •
What are the three basic types of spectra? Continuous Spectrum Emission Line Spectrum Absorption Line Spectrum Spectra of astrophysical objects are usually combinations of these three basic types.
Introduction to Spectroscopy
Three Types of Spectra Illustrating Kirchhof's Laws
Continuous Spectrum
Emission Line Spectrum
Absorption Line Spectrum
Chemical Fingerprints • Downward transitions produce a unique pattern of emission lines.
Production of Emission Lines
Chemical Fingerprints • Because those atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.
Production of Absorption Lines
Production of Emission Lines
Composition of a Mystery Gas
Thought Question Which letter(s) labels absorption lines? A B C D E
Thought Question 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
Properties of Thermal Radiation 1. Hotter objects emit more light at all frequencies per unit area. 2. Hotter objects have shorter wavelength of highest intensity
Wien’s Law Wien’s Laws
Thought Question Which is hotter? A blue star • A red star • A planet that emits only infrared light •
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.
Interpreting 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 K
What is this object? Carbon Dioxide: Absorption lines are the fingerprint of CO 2 in the atmosphere
What is this object? Ultraviolet Emission Lines: Indicate a hot upper atmosphere
What is this object? Mars!
How does light tell us the speed of a distant object? The Doppler Effect
The Doppler Effect Hearing the Doppler Effect as a Car Passes
Explaining the Doppler Effect Understanding the Cause of the Doppler Effect
Same for light The Doppler Effect for Visible Light
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.
The amount of blue or red shift tells us an object’s speed toward or away from us: The Doppler Shift of an Emission-Line Spectrum
Doppler shift tells us ONLY about the part of an object’s motion toward or away from us. How a Star's Motion Causes the Doppler Effect
Thought Question I measure a line in the lab at 500.7 nm. The same line in a star has wavelength 502.8 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. •
Measuring Redshift The Doppler Shift of an Emission-Line Spectrum
Measuring Redshift Doppler Shift of Absorption Lines
Measuring Velocity Determining the Velocity of a Gas Cloud
Measuring Velocity Determining the Velocity of a Cold Cloud of Hydrogen Gas
How do telescopes help us learn about the universe? • light-collecting area • angular resolution • Other frequencies (e.g., infrared, ultraviolet)
Bigger is better 1. Larger light-collecting area 2. Better angular resolution
Bigger is better Light Collecting Area of a Reflector
Angular Resolution • The minimum angular separation that the telescope can distinguish Angular Resolution Explained using Approaching Car Lights
Angular resolution: smaller is better Effect of Mirror Size on Angular Resolution
Basic Telescope Design • Refracting: lenses Refracting telescope Yerkes 1-m refractor
Basic Telescope Design • Reflecting: mirrors • Most research telescopes today are reflecting Reflecting telescope Gemini North 8-m
Mauna Kea, Hawaii
Different designs for different wavelengths of light Radio telescope (Arecibo, Puerto Rico)
Want to buy your own telescope? • Buy binoculars first (e.g., 7 × 35) — you get much more for the same money. • Ignore magnification (sales pitch!) • Notice: aperture size, optical quality, portability • Consumer research: Astronomy , Sky & Telescope , Mercury magazines; Astronomy clubs.
Why do we put telescopes into space? It is NOT because they are closer to the stars! Recall our 1-to-10 billion scale: • Sun size of grapefruit • Earth size of a tip of a ball point pen,15 m from Sun • Nearest stars 4,000 km away
Observing problems due to Earth’s atmosphere 1. Light Pollution
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