Thursday, February 21, 2013

Chapter 5 Notes: Olivia Ward

 Light and Matter: Reading Messages from the Cosmos

Basic Properties of Light and Matter & Telescopes
  • Light that comes from the Sun: it's white light because it's a blend of each light spectrum (colors).
  • Electromagnetic Energy Spectrum, spectrum, energy
  • Shorter wavelengths = higher energy, longer wavelengths = lower energy
  • Visible surface of the Sun: the Photosphere
  • The Earth is a cool body emitter: infrared heat
  • Collecting light with telescopes
    • Basic telescope design:
      • Refraction: prism telescope
    • Telescopes
      • Refractors: lenses bend light
        • Galileo
      • Reflector: mirrors
        • Most research telescopes today are reflecting
        • Hubble: UV and visible
      • X-Ray telescopes
        • To study black holes: Chandra
 5.1 Light in Everyday Life
How do we experience light?

  • The warmth of sunlight tells us that light is a form of energy.
  • Measuring the flower of energy in light: 1 watt = 1 joule/s
  • Colors of Light
    • White light is made up of many different colors.
How do light and matter interact?
  • Emission (fusion)
  •  Absorption (heat up matter)
  • Transmission (movement of electromagnetic matter through a medium)
    • Transparent objects transmit light.
    • Opaque objects block (absorb) light.
  • Reflection (changes its direction without changing its wavelength or giving up energy)
    • Albedo: the reflective quality of an object
  • Scattering (light reflection: changes direction without using up energy in any direction and allows us to see it)
    • Raleigh Scattering: shorter wavelengths experience greater scattering. (Longer wavelengths experience less scattering.)
  • Reflection and Scatter Example:
    • Mirrors reflect lights in a particular direction.
    • Movie screens scatter lights in all directions.
  • Interactions of light with matter
    • Interactions between light and matter determine the appearance of everything around us.
  • Question: Why is a rose red?
    • A. The rose absorbs red light.
    • B. The rose transmits red light.
    • C. The rose emits red light.
    • D. The rose reflects red light.

5.2 Properties of Light
What is light?
  • Light can act either like a wave or like a particle (photons).
  • Waves: A wave is a pattern of motion that can carry energy without carrying matter along with it.
  • 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
  • Light: Electromagnetic Waves
    • A light wave is a vibration of electric and magnetic fields.
    • Light is an electromagnetic wave.
    • Energy: anything that performs work
      • Electromagnetic
      • Gravity
      • Weak (subatomic)
      • Strong (subatomic)
    • Light interacts with charged particles through these electric and magnetic fields.
  • Wavelength and Frequency
    • wavelength X frequency = speed of light = constant
      • If the wavelength shortens, the frequency goes up = more energy
      • Speed of light: 300,000 km/sec
  • 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.
  • Special Topic: Polarized Sunglasses
    • Polarization describes the direction in which a light wave is vibrating.
    • Reflection can change the polarization of light.
    • Polarized sunglasses block light that reflects off of horizontal surfaces.
What is the electromagnetic spectrum?
  • Question: The higher the photon energy...
    • A. The longer its wavelength.
    • B. The shorter its wavelength.
    • C. Energy is independent of wavelength.

5.3 Properties of Matter
What is the structure of matter?
  • Molecules ← Elements ← Atoms ← Subatomic Particles: protons, neutrons, electrons ← Quarks
  • Conservation of Momentum
    • The total momentum of interacting objects cannot change unless an external force is acting on them.
    • Interacting objects exchange momentum through equal and opposite forces.
  • Atomic Terminology
    • Atomic number = # of protons in nucleus
    • Atomic mass number = # of protons + neutrons
    • Example: Carbon
      • Atomic number = 6
      • Atomic mass number = 12
      • 6 electrons
    • Molecules: consist of two or more atoms
    • Isotope: the same # of protons but different # of neutrons
  What are the phases of matter?
  • Familiar phases:
    • Solid
    • Liquid
    • Gas
  • Phases of same material behave different because of differences in chemical bonds.
    • Thermal energy is related to temperature but is not the same. Temperature is the average kinetic energy of the many particles in a substance.
    • Thermal energy is a measure of the total kinetic energy of all the particles in a substance. It therefore depends on both temperature and density.
  • Phase Changes
    • Ionization: stripping of electron, changing atoms into plasma
    • Dissociation: breaking of molecules into atoms
    • Evaporation: breaking of flexible chemical bonds, changing liquid into solid
    • Melting: breaking of rigid chemical bonds, changing solid into liquid
  • Phases and Pressure
    • Phase of a substance depends on both temperature and pressure
    • Often more than one phase is present
  • Conservation of Energy
    • Energy can be neither created nor destroyed.
    • It can change form or be exchanged between objects.
How is energy stored in atoms?
  • Energy Level Transitions
    • The only allowed changes in energy are those corresponding to a transition between energy levels.


5.4 Learning from Light
What are the three basic types of spectra?

  • Three Types of Spectra
    • Continuous Spectrum
      • The spectrum of a common (incandescent) light bulb spans all visible wavelengths, without interruption.
    •  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.
    • 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.
How does light tell us what things are made of?
  •  Chemical Fingerprints
    • Each type of atom has a unique set of energy levels.
    • Each transition corresponds to a unique photon energy, frequency, and wavelength.
    • Downward transitions produce a unique patter of emission lines.
    • Because atoms can absorb photons with those same energies, upward transitions produce a pattern of absorption lines at the same wavelengths.
    • Each type of atom has a unique spectral fingerprint.
      • Observing the fingerprints in a spectrum tells us which kinds of atoms are present.
  • Energy Levels of Molecules
    • Molecules have additional energy levels because they can vibrate and rotate.
    • The large number of virbrational and rotational energy levels can make the spectra of molecules very complicated.
    • Many of these molecular transitions are in the infrared part of the spectrum.
  • Question: Which letter(s) label(s) absorption lines?
    • Answer: C & D
  •  Question: Which letter(s) label(s) the peak (greatest intensity) of infrared light?
    • Answer: E
  • Question: Which letter(s) label(s) emission lines?
    • Answer: A
How does light tell us the temperatures of planets and stars?
  • Thermal Radiation
    • Nearly all large or dense objects emit thermal radiation, including stars, planets, and people.
  • Properties of Thermal Radiation (Wien's Law)
    • Hotter objects emit more light at all frequencies per unit area.
    • Hotter objects emit photons with a higher average energy.
  • Question:Which is hottest?
    • A. A blue star
    • B. A red star
    • C. A planet that emits only infrared light
  • Why don't we glow in the dark?
    • A. People do not emit any kind of light.
    • B. People only emit light that is invisible to our eyes.
    • C. People are too small to emit enough light for us to see.
    • D. People do not contain enough radioactive material.
How do we interpret an actual spectrum?
  • By carefully studying the features in a spectrum, we can learn a great deal about the object that created it.

5.5 The Doppler Shift 

How does light tell us the speed of a distant object?
  • The Doppler Effect
    • We generally measure the Doppler effect from shifts in the wavelengths of spectral lines.
    • The amount of blueshift or redshift tells us an object's speed toward or away from us.
    • Doppler shift tells us only about the part of an object's motion toward or away from us.
  • Question: I measure a line in a lab at 500.7 nm. The same line in a star has a 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 Blueshift (objects moving toward us)
  • Measuring Redshift  (objects moving away from us)
  • Measuring Velocity
How does light tell us the rotation rate of an object?
  • Different Doppler shifts from sides of a rotating object spread out its spectral lines.
  • Spectrum of a Rotating Object
    • Spectral lines are wider when an object rotates faster.

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