THE NATURE OF LIGHT

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THE NATURE OF LIGHT

• Light is an ELECTROMAGNETIC WAVE
• Light is also a PARTICLE the PHOTON
• This nominal contradiction is an example of
• COMPLEMENTARITY or DUALITY in QUANTUM MECHANICS.
  Depending on
  circumstances it is preferable to use one or the
  other point of view for light,
  electrons, protons, atoms anything which is too
  small to be described by normal physics and
  directly experience requires using quantum
  mechanics

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First, the Wave Nature

• Light is a TRANSVERSE ELECTROMAGNETIC
  WAVE.
• Electric (E) fields oscillate perpendicular to
• Magnetic (B) fields and the ENERGY FLOWS
  PERPENDICULAR to both fields.
• Other transverse waves WATER waves,
  SECONDARY (shear) SEISMIC waves.
• LONGITUDINAL WAVES have ENERGY FLOWS PARALLEL to
  oscillations SOUND, PRIMARY (compressional)
  SEISMIC waves.

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Transverse Electromagnetic (EM) Wave
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Electric and Magnetic Fields

• Charged particles (protons or ions , electrons
  -) attract or repel each other.
• Electric fields accelerate charged particles
  along the lines.
• Charged particles orbit around magnetic field
  lines.

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Characteristics of All Waves

• Frequency (f or ? nu) oscillations per sec
  (Hz)
• Speed (v) depends on medium, sometimes ? (cm/s
  or m/s)
• Wavelength (? lambda) distance between crests
  (cm)
• Amplitude (A) strength of oscillation
• Anatomy of a Wave Applet

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Key Relations for ALL Waves

• Speed Wavelength x Frequency v
  ?f
• Or f v/ ? or ? v/f
• Power ?Amplitude2 P ? A2
• Surface Waves in a Pond

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Wavelength and Frequency for Light

• wavelength x frequency speed of light constant

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For EM Waves ONLY

• v c 2.9979x108 m s-1 3.00x1010 cm s-1
• 3.00x105 km s-1 186,000 miles/s
• The speed of light does not depend upon direction
  or frequency in a vacuum and doesnt need a
  medium.
  It is a CONSTANT of
  NATURE.
• In matter, v lt c and the same frequency has a
  shorter wavelength than in vacuum.
  The INDEX OF REFRACTION,
  n c/v gt 1. In air, n 1.0003
  in normal glass, n ? 1.5

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Special Topic Polarized Light

• EM WAVES CAN ALSO BE POLARIZED
• E field in a particular plane B field in one
  perpendicular plane ? LINEAR POLARIZATION
  this is the only kind of polarization we'll worry
  about.
• Reflection can change the polarization of light
• Polarized sunglasses block light that reflects
  off of horizontal surfaces

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The Seven Bands of the EM Spectrum
? Microwave or millimeter between Radio and IR
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Atmospheric Transmission

• Radio ? gt 1 cm -- The longest waves or lowest
  frequencies.
• Penetrates atmosphere if ?lt15 m
• So AM reflects off ionosphere while FM penetrates
• Millimeter or microwave 1 cm gt ? gt 0.003 cm --
  partially penetrates atm molecules absorb.
• Infrared (IR) 0.003 cm gt ? gt 7.2x10-5 cm
  720 nm
• CO2 , H2O etc absorb most but some ?s penetrate

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Visible Wavelengths

• VISIBLE (OPTICAL)
  720 nm 7200 Å gt ? gt 380 nm 3800 Å,
  4.2 x 1014 Hz lt f lt 7.9 x 1014 Hz.
• Penetrates atmosphere (shorter scatter more)
• Visible spectrum RED (longest wavelength),
  ORANGE, YELLOW, GREEN, BLUE, VIOLET (shortest
  wavelength -- highest frequency)
• Our eyes evolved to see this light, since the Sun
  produces most of its radiation in this band, and
  since nearly all of this radiation gets through
  the atmosphere.
• Visible Light Applet

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Colors of Light

• White light is made up of many different colors

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Shortest Wavelengths

• ULTRAVIOLET (UV) 380 nm gt ? gt 300Å 30nm
  Mostly absorbed in atmosphere ozone (O3)
  Good thing, since UV radiation causes skin
  cancer.
• X-RAY 300 Å gt ? gt 0.1 Å 0.01nm,
  Absorbed in atmosphere by any atom
  (N, O) A good thing too X-rays can
  penetrate the body and cause cancer in many
  organs.
• GAMMA-RAY (?-ray) ? lt 0.1 Å 0.01 nm,
  The most energetic form of EM
  radiation. Absorbed high in atmosphere by
  any atomic nucleus.
  A VERY good
  thing gamma-rays quickly cause severe
  burns and cancer.

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Blue light is (compared to red light),

1. Shorter wavelength
2. Longer wavelength
3. Higher energy photons
4. 1 and 3
5. None of the above

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Blue light is (compared to red light),

1. Shorter wavelength
2. Longer wavelength
3. Higher energy photons
4. 1 and 3
5. None of the above

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We cant see infrared, but we can perceive it as

1. Heat
2. Radar
3. Sound
4. AM
5. FM

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We cant see infrared, but we can perceive it as

1. Heat
2. Radar
3. Sound
4. AM
5. FM

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How are Electromagnetic Waves Made?

• Most come from
  ATOMIC,
  MOLECULAR or
  NUCLEAR
  TRANSITIONS. I.e.,
  electrons or protons changing quantum states.
  
• BUT FUNDAMENTALLY, EM RADIATION IS PRODUCED BY
  AN ACCELERATED CHARGED PARTICLE.
• Since ELECTRONS have the LOWEST MASSES they are
  MOST EASILY ACCELERATED, therefore, electrons
  produce most EM waves.

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Examples of EM Wave Generation

• Radio - TV - Cell Phone transmission towers.
  Electrons oscillate up down
• Synchrotron Radiation produced by electrons
  spiraling around magnetic field lines, when
  moving at nearly speed of light.
• The circular part of the motion is ACCELERATED
  and produces the radiation. Synchrotron
  radiation is strongly POLARIZED most EM
  radiation is basically UNPOLARIZED

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How do Waves Interact with Matter?

• EMIT (light is sent out when a bulb is turned on)
• REFLECT (angle of incidence angle of
  reflection) or Scatter (spread out reflection)
• TRANSMIT (low opacity)
• ABSORB (high opacity)
• REFRACT (bend towards normal when entering a
  medium with a slower propagation speed)
• INTERFERE (only a WAVE can do this) Either
  CONSTRUCTIVE (waves add when in phase)
  DESTRUCTIVE (waves cancel when out of
  phase)
• DIFFRACT (only a WAVE can do this)
  Waves spread out when passing through a hole
  or slit. This is important only if the size of
  the hole or slit is comparable to the
  wavelength.

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Reflection and Scattering
Mirror reflects light in a particular direction
Movie screen scatters light in all directions
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Interactions of Light with Matter
Interactions between light and matter determine
the appearance of everything around us objects
reflect some wavelengths, absorb others and emit
others.
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When light approaches matter, it can

1. Be absorbed by the atoms in the matter
2. Go through the matter, and be transmitted
3. Bounce off the matter, and be reflected
4. Any of the above
5. Only 2 or 3

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When light approaches matter, it can

1. Be absorbed by the atoms in the matter
2. Go through the matter, and be transmitted
3. Bounce off the matter, and be reflected
4. Any of the above
5. Only 2 or 3

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Thought QuestionWhy is a rose red?

1. The rose absorbs red light.
2. The rose transmits red light.
3. The rose emits red light.
4. The rose reflects red light.

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Thought QuestionWhy is a rose red?

1. The rose absorbs red light.
2. The rose transmits red light.
3. The rose emits red light.
4. The rose reflects red light.

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Interference and Diffraction
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Light as Particles

• ELECTROMAGNETIC ENERGY IS CARRIED BY PHOTONS
• A PHOTON is a SINGLE QUANTUM OF LIGHT.
  The energy of one photon
  of a particular frequency is
• E hf h c / ?
  h 6.63 x
  10-34 Joule sec 6.63 x 10-27 erg sec is
  PLANCK's CONSTANT.
• Along with c, the speed of light
  e, the charge on an electron
  (or proton) and G (Newton's constant of
  gravity), h is one of the
• FUNDAMENTAL CONSTANTS of NATURE.

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Thought QuestionThe higher the photon energy

1. the longer its wavelength.
2. the shorter its wavelength.
3. energy is independent of wavelength.

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Thought QuestionThe higher the photon energy

1. the longer its wavelength.
2. the shorter its wavelength.
3. energy is independent of wavelength.

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Photons vs. Waves

• These PHOTONS can equally well explain
• REFLECTION,
• REFRACTION,
• TRANSMISSION and
• ABSORPTION
• as can the Wave picture,
• BUT they can't explain
• INTERFERENCE and
• DIFFRACTION.

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Photons vs. Waves, Round 2

• On the other hand the WAVE picture can't explain
• The PHOTOELECTRIC EFFECT
• (where metals emit electrons when
  light shines on them)
• and SPECTRAL LINES
• (where only specific wavelengths of light
  emerge from particular elements)
• while the PARTICLE part of the duality in Quantum
  Mechanics CAN!
• Well soon discuss each of these key aspects of
  light the latter is at the core of modern
  astronomy.

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How can light behave as both a wave and a
particle?

1. It doesnt really
2. It really is simultaneously both a wave and a
   particle
3. Light and small objects such as atoms behave in
   ways we never see in everyday objects, so we
   cant describe them in everyday terms
4. This is what quantum mechanics describes
5. 3 and 4

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How can light behave as both a wave and a
particle?

1. It doesnt really
2. It really is simultaneously both a wave and a
   particle
3. Light and small objects such as atoms behave in
   ways we never see in everyday objects, so we
   cant describe them in everyday terms
4. This is what quantum mechanics describes
5. 3 and 4

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Radiation, Temperature and Power

• Crudely, hotter matter produces more highly
  accelerated charged particles, which therefore
  produces more powerful EM radiation.
• Heat energy is proportional to temperature
• E k T
  (where T is in Kelvins, 0 at
  ABSOLUTE ZERO).
• So the thermal (heat) energy in atoms should be
  proportional to the photon energy using math
• h f ? kT OR
• ? ? 1/T

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Temperature Scales

• Only the US has stuck with Fahrenheit
  temperatures
• The rest of the world normally uses Celcius, but
• ENERGIES VANISH AT ABSOLUTE ZERO THE NATURAL
  TEMPERATURE SCALE IS KELVINS.
• The size of 1 degree C 1 K and 1.8 degrees F.
• 0 C 273.16 K (round it off)
• The conversion formula is F (9/5)C 32
• or C (5/9)(F - 32)

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Wiens Law

• Or, ?max2,900,000/T (nm)
• This is the PEAK WAVELENGTH for BLACKBODY (or
  Thermal, or Planckian) emission from a SOLID, a
  LIQUID or a DENSE GAS.
• Ex T 5800K 5.8x103K

0.5x10-4cm5x10-5cm 500 nm 5000Å
Wien's Law Applet
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Thermal Spectra
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Properties of Thermal Radiation

• Hotter objects emit more light at all frequencies
  per unit area.
• Hotter objects emit photons with a higher average
  energy.

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Thought QuestionWhich is hotter?

1. A blue star.
2. A red star.
3. A planet that emits only infrared light.

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Thought QuestionWhich is hotter?

1. A blue star.
2. A red star.
3. A planet that emits only infrared light.

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Thought QuestionWhy dont we glow in the dark?

1. People do not emit any kind of light.
2. People essentially only emit light that is
   invisible to our eyes.
3. People are too small to emit enough light for us
   to see.
4. People do not contain enough radioactive material.

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Thought QuestionWhy dont we glow in the dark?

1. People do not emit any kind of light.
2. People essentially only emit light that is
   invisible to our eyes.
3. People are too small to emit enough light for us
   to see.
4. People do not contain enough radioactive material.

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The Photoelectric Effect

• Electrons can be expelled from many materials if
  light shines upon them.
• If the wavelength is TOO LONG (low frequency)
  nothing happens,
• EVEN IF the INTENSITY of the light is HIGH.
• Above a CRITICAL FREQUENCY the emitted
  electrons have a maximum energy (or velocity)
  that RISES with the FREQUENCY.
• Ee h f - h fcrit
• Increasing the INTENSITY of light above the
  critical frequency increases only the number of
  ejected electrons, but NOT their energies.

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Photoelectric Effect, Illustrated
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Importance of Photoelectric Effect

• Einstein pointed out that the wave theory could
  not explain this, while quanta of energy, with
  E h f could.
• The wave theory predicted that even red light, if
  intense enough, would eject electrons --
  but this never happened.
• The wave theory also said that as the blue light
  was made brighter, faster electrons would
  emerge
• instead only more of them came out,
• but their maximum kinetic (motion) energy was a
  function ONLY of the light's FREQUENCY.

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The Stefan-Boltzmann Law

• Integrate (add up) a blackbody spectrum and find
  that the FLUX, or ENERGY/TIME/AREA is given by
• F ? T4
  where ? 5.67x10-8W m-2 K-4
  5.67x10-5erg s-1cm-2K-4
• POWER FLUX x AREA, or, for a sphere (of
  AREA 4 ? R2)
  L 4 ? ? R2 T4
• For example, double T and raise L 16 times!
  Or raise T by 1 and L goes up about 4.

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Peer Instruction QuestionWhat happens to
thermal radiation (a continuous spectrum) if you
make the source hotter?

• More energy comes out at all wavelengths
• The peak of the spectrum-energy curve (the
  wavelength at which most energy is emitted)
  shifts redward
• The peak of the spectrum-energy curve shifts
  blueward
• 1 and 2
• 1 and 3

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What happens to thermal radiation (a continuous
spectrum) if you make the source hotter?

• More energy comes out at all wavelengths
• The peak of the spectrum-energy curve (the
  wavelength at which most energy is emitted)
  shifts redward
• The peak of the spectrum-energy curve shifts
  blueward
• 1 and 2
• 1 and 3

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Luminosity, Temperature and Size

• Since T can be measured by Wien's Law and L can
  be obtained from the star's or planets
  brightness and distance (we'll discuss later)
  this formula lets astronomers find the SIZES of
  planets and stars!
• Well do that later for now lets compare
  luminosities
• Example T1 500 K, R1 2,000 km
• T2 250 K, R2 4,000 km
• L1/L2 (2,000 km/4,000 km)2 (500 K/250 K)4
• (1/2)2 (2)4 (1/4) 16 4 or L1/L24
• In words, Planet 1 has 4 times the luminosity of
  Planet 2

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WHAT IS THE STRUCTURE OF MATTER?
Electron Cloud
Atom
Nucleus
Nucleus size only around 10-15m while electron
clouds are roughly 10-10m 0.1 nm 1Å As nearly
all of the mass is in the nucleus, matter is
mostly empty space!
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Atomic Terminology

• Atomic Number of protons in nucleus
• Atomic Mass Number of protons neutrons

• Molecules consist of two or more atoms (H2O,
  CO2)

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Atomic Terminology

• Isotope same of protons but different of
  neutrons. (4He, 3He)

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What is found in the nucleus of atoms?

1. Protons with a charge
2. Neutrons with no charge
3. Electrons with a charge
4. All of the above
5. 1 and 2

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What is found in the nucleus of atoms?

1. Protons with a charge
2. Neutrons with no charge
3. Electrons with a charge
4. All of the above
5. 1 and 2

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How is the isotope 14C different from 12C?

• It has more protons
• It has more neutrons
• It has more electrons
• All of the above
• None of the above

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How is the isotope 14C different from 12C?

• It has more protons
• It has more neutrons
• It has more electrons
• All of the above
• None of the above

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What are the phases of matter?

• Familiar phases
• Solid (ice)
• Liquid (water)
• Gas (water vapor)
• Phases of same material behave differently
  because of differences in chemical bonds
• Less familiar phase ionized gas, where electrons
  are ripped off atoms.
• But this is most of the normal matter in the
  Universe!

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Phase Changes

• Ionization Stripping of electrons, changing
  atoms into plasma
• Dissociation Breaking of molecules into atoms
• Evaporation Breaking of flexible chemical bonds,
  changing liquid into gas
• Melting Breaking of rigid chemical bonds,
  changing solid into liquid
• Sublimation from solid to gas

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Review of the Nature of Matter

• What is the structure of matter?
• Matter is made of atoms, which consist of a
  nucleus of protons and neutrons surrounded by a
  cloud of electrons
• What are the phases of matter?
• Adding heat to a substance changes its phase by
  breaking chemical bonds.
• As temperature rises, a substance transforms from
  a solid to a liquid to a gas, then the molecules
  can dissociate into atoms
• Stripping of electrons from atoms (ionization)
  turns the substance into a plasma