Download Lecture 6

Lecture 6:
The Physics of Light, Part 1
Astronomy 111
Wednesday September 14, 2016
Reminders
• Star party
Thursday night!
• Homework #3 due
Monday
• Exam #1
Wednesday,
September 21
ASTR111 Lecture 6
The nature of light
―Look, but don’t touch.‖
- Astronomers’ Motto
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Visible light is just one form of
electromagnetic radiation
The universe contains electrically
charged particles: electrons (-) and
protons (+).
Charged particles are surrounded by
electric fields and magnetic fields.
Fluctuations in these fields produce
electromagnetic radiation.
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Visible light is just one form of
electromagnetic radiation
- but so are
radio waves,
microwaves,
infrared light,
ultraviolet light,
X-rays, and
gamma rays.
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Speed of light
Speed of wave, c, equals wavelength
times frequency (units = meter/sec):
c=lxn
The speed of light in a vacuum is always
c = 300,000 km/s
(186,000 miles/sec).
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Speed of light
Ole Romer (Danish, 1644-1710)
was the first person to measure the
speed of light
Measured timing of eclipses of
Jupiter’s moon Io at different times
of the year—observed that light
took longer when Earth was near
Jupiter’s orbit!
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Light year
• A light-year is the distance light travels in one
year
• 1 light-year = 9.5 x 1012 km
• A unit of distance—not a unit of time!
• For reference,
– The Moon is 1.25 light-seconds from Earth
– Earth is 8.3 light-minutes from the Sun
– The Sun is 4.3 light-years from the nearest star
ASTR111 Lecture 6
Light can be thought of as a
wave
Wave = a periodic fluctuation travelling through
a medium.
Ocean wave = fluctuation in the height of water.
Sound wave = fluctuation in air pressure.
Electromagnetic wave = fluctuation in electric
and magnetic fields.
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Wave characteristics
(1) Wavelength, l (lambda): distance
between wave crests (units = meter).
(2) Frequency, n (nu): number of crests
passing per second (units = 1/sec =
Hertz).
(3) Amplitude, a: height of wave crests.
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Wave characteristics
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Particle nature of light
• Particles of light are called ―photons‖
• Each photon has a wavelength and a frequency
• A photon’s energy depends on its frequency
(wavelength)
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Photons
The energy of a photon is related to the
frequency of a wave:
E = hf
E = energy of photon
f = frequency of light (also called n)
h = Planck’s constant
(A Small Number)
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Photons
Don’t forget units!
Wavelength -> length
Frequency -> 1/time (per second)
Energy -> joules
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Light forms a spectrum from
short to long wavelengths
Visible light has wavelengths from 400
to 700 nanometers. [1 nanometer (nm)
= 10-9 meter]
Color is determined by wavelength:
Blue: 480 nm
Green: 530 nm
Red: 660 nm
ASTR111 Lecture 6
ASTR111 Lecture 6
The complete spectrum of light
Gamma rays (l < 0.01 nanometers)
X-rays (0.01 – 10 nm)
Ultraviolet (10 – 400 nm)
Visible (400 – 700 nm)
Infrared (700 nm – 1 mm)
Microwaves (1 – 100 mm)
Radio (> 100 mm)
Energy
Visible
light
occupies
only a tiny
sliver of
the full
spectrum.
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ASTR111 Lecture 6
Earth’s atmosphere is
transparent to visible light and
some microwaves and radio
waves.
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To observe efficiently at other wavelengths, we
must go above atmosphere.
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NASA's SOFIA Observatory flies a 2.7 m
telescope to altitudes as high as 45,000 feet.
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Sky: Optical
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Sky: Infrared
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Sky: Microwaves
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Sky: Radio
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Sky: X-ray
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How light and matter interact
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Atoms
Ordinary matter is found primarily in the
form of atoms.
Range of ordinary matter:
– free subatomic particles (protons &
electrons)
– single atoms (hydrogen, helium, gold, etc.)
– simple molecules (O2, H2O)
– macromolecules (DNA, complex polymers)
ASTR111 Lecture 6
Atomic structure
Nucleus of heavy subatomic particles:
– proton: positively charged
– neutron: uncharged (neutral)
Cloud of Electrons orbiting the Nucleus:
– electron: negatively charged.
– mass 1/1860th of proton
Mostly empty space
1 part in 1015 of the volume is occupied.
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Simple atoms
1H
4He
proton
electron
neutron
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Chemical elements
Distinguish atoms into Elements by the
total number of protons in the nucleus.
1 proton = Hydrogen
2 protons = Helium
3 protons = Lithium ... and so on
Number of electrons = Number of protons
(at least in conditions here on earth)
Elements are Chemically Distinct
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Isotopes
Distinguish elements into Isotopes by the
number of neutrons in the nucleus.
Example:
12C
has 6 protons and 6 neutrons
13C has 6 protons and 7 neutrons
14C has 6 protons and 8 neutrons
same # of protons & electrons, but
different # of neutrons
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Hydrogen
1 proton
1H
2H
3He
4He
Helium
2 protons
Lithium
3 protons
6Li
Proton:
7Li
Neutron:
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3H
Radioactivity
If too many or too few neutrons in a
nucleus, it is unstable against
radioactive decay.
Examples:
(1p+2n)  3He (2p+1n) + e- + ne
14C (6p+8n)  14N (7p+7n) + e- + n
e
3H
(basis of radioactive carbon dating)
Free neutrons are unstable:
n  p + e- + ne
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Energy stored in atoms and
molecules emit or absorb light
Consider a single,
isolated atom:
A nucleus, containing
protons and
neutrons, is
surrounded by a
cloud of orbiting
electrons.
Electrons can emit or
absorb photons.
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Consider hydrogen (the simplest
atom): one proton, one electron
Behavior on subatomic
scales is governed by
quantum mechanics.
One rule of quantum
mechanics: electrons
can only exist in orbits of
particular energy (energy
is quantized).
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Emission & absorption
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Excitation
Start out in the Ground State:
All electrons are in their lowest energy orbits.
To excite an electron into a higher energy
orbit, you need to absorb exactly the
energy difference between orbits:
– absorb a photon of exactly that energy
– collide with an atom or electron and get the
energy from the motion of the collider.
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Absorb a photon
photon
Collide with an electron
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Absorption
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De-excitation
Excited states are unstable, and electrons
will decay back into their ground states.
To de-excite, an electron must rid itself of
exactly the amount of excess energy:
– emit a photon of the exact energy.
– give up the energy to a colliding atom or
electron (no photons are emitted).
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Emit a photon
photon
Collide with an electron
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Emission
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Line spectra
• Electrons can only orbit in discrete
Energy Levels.
• Atoms & molecules can only emit or
absorb photons at particular
wavelengths.
– a unique ―line spectrum‖ for each type of
atom or molecule.
– what lines you see depends on the state of
excitation and ionization of the system.
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Emission & absorption lines
• Emission lines
Photons emitted at particular wavelengths
when an electron jumps from a higher to a
lower energy orbit.
• Absorption lines
Photons absorbed at particular wavelengths
if their energy is exactly enough to make
an electron jump up to a higher energy
orbit.
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Emission & absorption lines
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Ionization
If an atom or molecule absorbs enough
energy from a photon or a collision, an
electron can be ejected.
Ion: positively charged atom or molecule.
– Changes the spectral line signature
– Changes the chemical properties
Distinguish ions by the number of
electrons removed.
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Absorb a photon
ion
ion
photon
Collide with an electron
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Fundamental forces of nature
All interactions in nature are governed by
4 ―fundamental‖ forces:
• Gravitational Force
• Electromagnetic Force
• Strong Nuclear Force
• Weak Nuclear Force
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Gravitational force
Gravitation binds masses over long
distances
• Long-range attractive force
• Weakest force of nature
• Obeys the Inverse Square Law of
distance:
m1m2
F = G
2
d
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Electromagnetic force
Acts between charged particles:
• like charges repel each other
• opposite charges attract each other
Long-range, inverse-square law force.
Binds:
• electrons to protons in atoms
• atoms to atoms in molecules
Very strong: 1040 times stronger than
Gravity.
ASTR111 Lecture 6
Strong & weak nuclear forces
Short-range forces (<10-15 m) in atomic
nuclei
Strong Force:
– binds protons & neutrons into nuclei.
– strongest force of nature.
Weak Force:
– responsible for radioactivity (turns neutron
into a proton)
– second weakest force.
ASTR111 Lecture 6
Interplay of forces
Gravity rules on the largest scales.
Electromagnetism rules on intermediate
scales (atomic scales up to people)
Strong & Weak Forces rule on nuclear
scales.
We will explore the different roles of each
in our study of stars, galaxies & the
Universe.
ASTR111 Lecture 6
Questions:
1) Why are our eyes sensitive to ―visible‖
light?
2) Could we have radio eyes?
3) Why is a leaf green?
ASTR111 Lecture 6