Chapter 5: Light and Matter

Let us start with Matter

         Matter is made of atoms

Components of an atom:
                  protons ("massive", + charge), neutrons("massive", no charge), electrons (lightweight, - charge)
                  nucleus = center area, where the protons & neutrons are
                           grouped together via a nuclear force called the "strong force"
                  opposite charges attract, similar charges repel each other
                  atoms with equal number of protons and electrons are electrically neutral

         The "type" of atom is determined by the number of protons
                  hydrogen atom has 1 proton, helium atom has 2 protons, etc.
                  "atomic number" = number of protons in atom
                  different "types" of atoms => different elements
example figure
periodic table of the elements

         Isotopes
                  Usually an atom has about same number of protons as neutrons, but not always
                  The different versions = different isotopes
                  Example = ¹²C: superscript = number of protons + neutrons = atomic mass number
Example

         Molecules:
                  Atoms can bond with other atoms ==> molecules
                  Ex: water = 2 hydrogen atoms + 1 oxygen atom = H₂O
Others: carbon monoxide (CO), H₂

"Phases of Matter" = gas, liquid, solid & plasma
microscopic view
                  Raising temperature changes phase: solid to liquid to gas to plasma
                  Reason: higher temperature -> faster collisions -> break bonds, ionize

Question:
Nearly all the matter in the Sun is in the plasma phase. What does this tell you about the Sun?
Do you think that plasma is common or rare on Earth?
Why would that be the case?

Energy Within atoms

"Potential Energy" of electrons within atoms
                  Opposite charges attract, similar charges repel each other
                  Electron that is further from proton (more smeared-out) =>
                            more electrical potential energy
                  Analogous to gravitational potential energy
                  If given enough energy, an electron can leave the atom
Pictorial Representation

Terminology
                  Ground state: electron is closest to nucleus, minimum electrical potential energy
                  Excited state: electron cloud is more spread out
                  Ionized: when electron is so distant from nucleus that it can roam freely

A Quantum Leap in Understanding

Contrast with our perception of large-scale world
                  Feels like can lift an object to any of a range of heights
                  So, feels like can have any of a range of gravitational potential energies
                  But! on atomic scale, electron can only have specific electrial potential energies
Energy Level Diagram

Terminology and Rules
                  Quantum levels: name for the "allowed" energy levels
                  If give right amount of energy to atom, electron can jump up
                  Electron can jump down to lower level and give off energy
                  If give an atom more energy than its highest level, then electron can leave
                  Ways to give atom energy = collisions, light
                  Ways for atom to give off energy = light, transfer energy to another atom

The transitions make signatures we can see and use in astronomy
Signature of hydrogen and another hydrogen energy level diagram
Constellation of Orion
Orion Nebula's red light from Hydrogen 3 -> 2 transition (called H alpha)

Question (problem #19):
How about these transitions?
                  a.) Which transition is an electron gaining 10.2 eV?
                  b.) Which transition is an electron losing 10.2 eV?
                  c.) Which transition is an electron breaking loose?
                  d.) Which displayed arrow is not an allowed transition?
                  e.) What is happening in transition A?

         Gas can emit and absorb or scatter light

Light

         Light is a form of energy (radiative energy)
                  Energy/time = power
                  Power units = watt = 1 Joule/sec
         White light is actually a combination of all colors of the rainbow
prism
         Light can be
                  emitted by an object (example = the Sun)
                  absorbed by an object (example = black board)
                  transmitted through an object (example = eye glasses)
                  reflected by an object (example = mirror)
         Most objects emit, absorb, transmit & reflect some colors
                  better than others
                  We see the color of the light that enters our eyes,
                  So, we see the colors emitted, transmitted, & reflected
example

Question:
         Some lights, such as neon lights don't emit the full spectrum of white light.
         If you looked carefully at a rainbow colored tie-dyed T shirt under a green light,
         would you see the normal rainbow? Why / Why not?

Light has seemingly contradictory properties
         Humans describe light like blind people describe an elephant
         One says the elephant is a wall, another thinks it is a spear, etc. (Saxe poem)
Light acts like both a particle and a wave
         Particle: light is made of individual photons
         Wave: light carries energy, but not material, just like a wave does
                  A wave passing through water causes the water to
                  rise and fall. Example
                  Similarly, as a light wave passes by, its magnetic and
                  electric fields increase and decrease
         Light has wavelength, frequency, speed, as do waves
                  Wavelength: distance between peaks
Frequency: number of peaks that pass by per second
                  Speed: speed that light wave propagates
                  speed = wavelength * frequency
         (Magnetic field is familar: it causes certain objects to move)
         (Electric field causes charged objects to move)
Electromagnetic wave

         Speed of light (in vacuum) = 300,000 km/sec = 3.0 x 10⁸ m/s
         (light travels slower when going through stuff)

         Different colors of light have different wavelenghts
                  and different energies (shorter wavelenghts -> more energy)
                  Energy = frequency * h
                           h = Planck's constant
                           h = 6.626 x 10^-34 Joule * sec
                           h = 4.136 x 10^-15 eV * sec

Problem
         Earlier we learned about the electrons inside atoms having varous energy "levels".
         What is the frequency of the light made when an electron moves from 12.1 eV to 10.2 eV ?
         What is the wavelength of the light produced ?
                  Hint: this light is red in color

Electromagnetic Spectrum
         The "visible spectrum" is just a small part of the whole spectrum
whole spectrum (even radio waves are a form of light)

Problem:
         Does this make sense? yes or no
         Because of their higher energies, X-rays must travel
         through space faster than radio waves. Yes or no?

Spectra from Space

         How light interacts with "optically thin" material:
                  a cloud is always emitting some light
                  you see cloud absorb light if it is between you and light source
                  you see reflection (scattering) if you are off line of sight

         different elements -> different "emission/absorption line spectra"
         different ionization levels (having lost or gained electrons)
                  -> different "emission/absorption line spectra"
Molecules: can rotate and vibrate, but at particular rates,
         so they make/absorb light of many colors
         spectra tell us the chemical composition, ionization level (temperature)        
                  Example: nebula = cloud in space and
                  the Orion nebula's ultraviolet spectrum

How Light interacts with "optically thick" (opaque) material:
         original photons are absorbed, new ones emitted, repeat many times
         result = a rainbow of photon energies called "continuous spectrum"
         example: spectra from stars
         radiation is called "thermal radiation" or "blackbody radiation"
         "blackbody radiation" depends on the temperature
                  1.) Hotter objects emit more energy/(surface area)
                           (Stephan - Boltzman Law)
                  2.) The brightest part of the rainbow shifts to shorter
                  wavelengths as the temperature increases
                           (Wien's Law)
         examples:
fire poker
spectrum versus temperature
animated version
human

Questions about this spectrum of a planet:
         Do you see the "line emission" from optically thin gas? Where in the spectrum?
         Do you see the blackbody emission? Where in the spectrum?
         If the planet has a thin atmosphere would you see absorption lines?
         Do you see absorption lines?
         What color does this planet look to the eye?

The Doppler Effect

Example: Car horn at 30 m.p.h.
Astronomy example: stars moving toward or away from us
Equation:         (radial velocity)/(speed of light)
                  = (observed wavelength - rest wavelength)/(rest wavelength)

radial velocity is not the same as tangential (transverse) velocity   
A star in an elliptical orbit
A rotating star
Measuring radial velocities from spectra: emission lines absorption lines

Question: Does this make sense, yes or no?
         If a distant galaxy has a substantial redshift (as viewed from our,
         galaxy then anyone living in that galaxy would see a substantial
         redshift in a spectrum of the Milky Way galaxy. Yes or No ?