Quiz 1

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Quiz 1

• Each quiz sheet has a different 5-digit
  symmetric number which must be filled in (as
  shown on the transparency, but NOT the same
  one!!!!!)
• Please hand in both the exam and the answer
  sheets with your name on both
• Question/answer sheets will be handed back on
  Wednesday after class
• Please remain seated until we begin collecting
  (20-25 minutes after start)
• Class after quiz

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Why is the sky blue ?
The atmosphere scatters the blue light more than
red light
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Light and Matter

• Light is electromagnetic energy, due to
  interaction of electrical charges
• Matter is made of atoms equal number of
  positive and negative particles
• An atom is the smallest particle of an element
  natural element H to U
• Atom ? Nucleus (protons neutrons), with
  orbiting electrons
• No. of protons in nucleus Atomic Number
• Science of light ? Spectroscopy

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Radiation and Spectroscopy

• Light is electromagnetic energy
• Propagates as both particles and waves
• Photons particles of light
• Wavelength Velocity / Frequency

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Light is electromagnetic waveDoes not require a
medium to propagate, unlike water or sound
Wavelength is the distance between successive
crests or troughs
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WAVES Frequency, Wavelength, Speed
Frequency (f) ( waves/second)
Frequency f is the number of waves passing a
point per second
Speed wavelength x frequency ? c
l f
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Units of wavelength and frequency

• Frequency is the number of cycles per second
• Since speed of light is constant, higher the
  frequency the shorter the wavelength and
  vice-versa
• Wavelengths are measured in Angstroms 1A
  1/100,000,000 cm 1/10 nanometer (nm)
• The higher the frequency the more energetic the
  wave
• Wavelength (or frequency) defines radiation or
  color

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Prisms disperse light into its component colors
Red-Violet
Prism
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Visible Light

• Forms a narrow band within the electromagnetic
  spectrum ranging from gamma rays to radio waves
• Human eye is most sensitive to which color?
• Yellow. Why?

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Light Electromagnetic SpectrumFrom Gamma Rays
to Radio Waves
Gamma X-Ray UV Visible
Gamma rays are the most energetic (highest
frequency, shortest wavelength), Radio waves are
the least energetic.
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Q1, AU15, A1143, Pradhan Curve 8
D
B
C
A
E
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Decreasing Wavelength OR Increasing Frequency
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Visible light spectrum Each color is defined by
its wavelength, frequency or energy
Red - Blue ? 7000 - 4000 Angstroms ( 1 nm
10 A, 1 A 10-8 cm) Blue light is more
energetic than red light Light also behaves like
particles called photons Photon energy,
frequency, wavelength E h f hc/l Plancks
Law (h is a number known as Plancks constant)
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Matter and Particles of Light Quantum Theory

• Light (energy) and matter in motion behave both
  as waves and particles
• Wave-Particle Duality - Quantum Theory
• Particles of light are called photons E hf
  hc/l
• Photons of a specific wavelength l may be
  absorbed or emitted by atoms in matter
• Matter is made of different natural elements
  lightest Hydrogen (1 proton), heaviest Uranium
  (92 protons)
• Smallest particle of an element is atom, made up
  of a nucleus (protons and neutrons), and orbiting
  electrons
• Electrons and protons attract as opposite
  electrical charges, NOT gravitationally like
  planets and Sun

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The simplest atom Ordinary Hydrogen
Resemblance to planets orbiting the Sun
is superficial !
Electrons also move both as particles and waves
p positively charged e negatively
One proton in the center (nucleus) and one
electron in orbits of definite energy Ordinary H
has no neutrons, but heavy hydrogen has one
neutron in the nucleus
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Absorption and emission of photons by H-atom
An electron may absorb or emit light photons at
specific wavelength
Wavelength (n 3 ? n 2) 6562 Angstroms (RED
Color)
Energy of the photon must be exactly equal to the
energy difference between the two orbits
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file///E/Univ7e/content/ch05/0503002.html
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Energy Level Diagram of 1H
Continuum
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Photons of all other energies (wavelengths) are
ignored and pass on by unabsorbed.
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Larger Jump More Energy Bluer Wavelength
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Series of spectral lines of Hydrogen
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Wavelengths of series of lines from Hydrogen
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Spectrum of a Fluorescent Light
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Characteristic spectra of elements
Each element has a unique set of spectral lines,
thus enabling its identification in the source.
Observations of spectra of different elements in
a source (planet, star, galaxy etc.) yields its
chemical composition
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Continuous, Absorption, and Emission spectra of a
source
Continuous spectrum covers wavelengths in a given
range absorption or emission spectrum consists
of dark or bright lines respectively at definite
wavelengths
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Brightness and Temperature

• Brightness is related to the total energy
  emitted, or the luminosity of an object
• The energy emitted is related to the temperature
  of the object
• B s T4 (s is a constant)
• Stefan-Boltzmann Law

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Color Indicates Temperature and Energy of the
Source
Blackbody Perfect absorber and emitter Of
radiation at a given Temperature T
Surface T (Sun) 5600 K (Mercury)
800 K
Objects generally emit radiation at all
wavelengths, but mostly at one peak Wavelength
depending on their temperature (e.g. blue hot,
red cool)
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TEMPERATURE SCALES
Astronomers usually use the Kelvin Scale
Room Temp 300 K 27 C 81 F
K C 273 C (F - 32) x 5/9 (F - 30)
/ 2 F (C x 9/5) 32 C x 2 30
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Brightness decreases inversely as the square of
the distance
B1
B1/4
B1/9
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The Doppler Effect

• Why does the pitch of a police siren differ
  when, say, a police car is approaching you, or
  when you are running away from the police (not
  recommended) ?
• The frequency (the number of sound waves per
  second) is higher when approaching, and smaller
  when receding from the source

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Doppler Effect in Sound
Low Pitch (long waves)
High Pitch (short waves)
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The Doppler Effect
Velocity c frequency (f) x wavelength (l)
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Doppler Shift of Wavelengths

• What about the wavelength?
• What about light?
• Shorter wavelength ? Blue-shift,
• Longer wavelength ? Red-shift
• We can determine the velocity of astronomical
  objects, moving away or towards the Earth, by
  measuring the wavelength of light from the object
• Observed red-shift of galaxies all over the sky
  shows that galaxies are moving away from one
  another ? the Universe is expanding (Hubbles
  Law)

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Hubble Diagram Distribution of Galaxies
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Hubbles Law v HodVelocity increases with
distance
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Expanding Universe

• Hubbles law ? Universe is expanding
• Universe had a beginning !
• How long ago? Distance/Velocity time !
• Ho 71 Km/sec/Mpc (units of v/d 1/t)
• Age of the universe 1/Ho (units of time)
• Big Bang !! About 13.7 billion years ago
• How does one determine distances?
• Redshift

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Cosmic Microwave Background (CMB) Universal
and Uniform Radiation
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CMB Properties

• The entire universe is filled with extremely
  uniform radiation
• CMB radiation corresponds to a fixed temperature
  of 2.73 K (-270.3 oC or -428.9 oF)
• Blackbody uniform temperature oven
• CMB radiation is also isotropic ? same in all
  directions
• But with extremely slight variations immediately
  following the Big Bang due to matter

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Distribution of Matter in Galaxy

• Stars rotate about the center of galaxy
• Velocity determined by gravity mass Mc and
  distance Rc from the center
• KE PE
• ½ mstar v2 G Mc mstar / Rc
• Velocity v should decrease with radius Rc
• Surprise !

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Evidence of Dark Matter Rotation Curves of
Galaxies
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Dark Matter Halo

• Rotation curves are flat out to distances beyond
  observable galaxies
• Ergo Galaxies have dark matter haloes
• What is dark matter?