# Chapter 5: Light and 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

## 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):
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?

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