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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
Why is the sky blue ?
The atmosphere scatters the blue light more than red light
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
Radiation and Spectroscopy
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Light is electromagnetic energy
Propagates as both particles and waves
Photons – particles of light
Wavelength = Velocity / Frequency
Light is electromagnetic wave;
Does not require a medium to propagate,
unlike water or sound
Wavelength is the distance between successive crests or troughs
WAVES: Frequency, Wavelength, Speed
Wavelength ()
Frequency (f)
(# waves/second)
Speed (c)
Frequency ‘f’ is the number of waves passing a point per second
Speed = wavelength x frequency

c=f
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 viceversa
• 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
Prisms disperse light into its
component colors: Red-Violet
White
Light
Spectrum
Prism
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?
Light: Electromagnetic Spectrum
From 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.
Q1, AU15, A1143, Pradhan – Curve: +8%
E
D
C
B
A
Decreasing
Wavelength
OR
Increasing
Frequency
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/
Planck’s Law (‘h’ is a number known as Planck’s
constant)
Matter and Particles of Light: Quantum Theory
• Light (energy) and matter in motion behave both as
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waves and particles
Wave-Particle Duality - Quantum Theory
Particles of light are called photons: E = hf = hc/
Photons of a specific wavelength  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
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
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’
file:///E:/Univ7e/content/ch05/0503002.html
Continuum
n=
n=5
n=4
Energy Level
Diagram of 1H
n=3 (2nd excited state)
n=2 (1st excited state)
n=1 (Ground State)
26 25
24
n=23
n=6
n=5
n=4
n=3 (2nd excited state)
n=2 (1st excited state)
Photons of all other energies (wavelengths) are
ignored and pass on by unabsorbed.
n=1 (Ground State)
62 52
42
n=32
n=6
n=5
n=4
n=3 (2nd excited state)
n=2 (1st excited state)
Larger Jump = More Energy = Bluer Wavelength
n=1 (Ground State)
Series of spectral lines of Hydrogen
Wavelengths of series of lines from Hydrogen
Spectrum of a Fluorescent Light
Mercury
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
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
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
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B = s T4 (s is a constant)
Stefan-Boltzmann Law
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)
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 = (C x 9/5) + 32
~ (F - 30) / 2
~ C x 2 + 30
Brightness decreases inversely as the square of the distance
d=1
B=1
d=2
B=1/4
d=3
B=1/9
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
Doppler Effect in Sound
High Pitch
(short waves)
Low Pitch
(long waves)
The Doppler Effect
Velocity c = frequency (f) x wavelength ()
Doppler Shift of Wavelengths
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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
(Hubble’s Law)
Hubble Diagram: Distribution of Galaxies
Hubble’s Law: v = Hod
Velocity increases with distance
Expanding Universe
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Hubble’s 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
Cosmic Microwave Background (CMB):
Universal and Uniform Radiation
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
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 !
Evidence of Dark Matter:
Rotation Curves of Galaxies
Dark Matter Halo
• Rotation curves are flat out to distances
beyond observable galaxies
• Ergo: Galaxies have “dark matter” haloes
• What is dark matter?