Light and Matter
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
"Phases of Matter" = gas, liquid, solid & plasma
Raising temperature changes phase: solid to liquid to gas to plasma Reason: higher temperature -> faster collisions -> break bonds, ionize
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?
"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
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 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
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.
wave on surface of coffee (You Tube, looped)
slow motion water drops with music, by Daniel Nimmervoll (You Tube, looped)
water and sound tricks with some music (You Tube, looped)
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
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
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?
Light associated with clouds of gas ("optically thin" material):
Example: Aurora light and spectrum Example: planetary nebula 2437 and spectrum, ngc 2437 different elements -> different "emission/absorption line spectra"
Excited atoms can emit photons of particular energies and absorb photons of the same particular energies Hydrogen atom example 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
example of Doppler shift in sound car horn (You Tube)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 ?