is light a wave or is light a particle !?!. experiment 1: experiment: light is incident on two...

Is light a wave or is light a

particle!?!

Experiment 1:

Experiment:

Light is incident on two closely spaced slits and the pattern is observed on a wall or screen across the room.

Young’s Double Slit Experiment

If light was a particle (think painted tennis balls)…

• what pattern would you see on the wall?

expectations

If light was a wave (think water)…

• what pattern would you see coming out of the double slits?

Double Slit Experiment

Light behaving like a wave.

Notice the light and dark fringes.

What do the dark spots represent?

What to the light spots represent?

What do the spots with no spots

represent?

Wave moving through a double slit and exhibiting

interference

Young’s Double Slit Experiment:

Shows that light

behaves like a wave

Experiment 2:

Experiment:

Light is incident on a piece of metal.

Photoelectric Effect

If light was a wave (think water)…

• How would the energy of a ocean wave (amplitude) affect the speed at which the electrons get kicked out?

If light was a particle (think painted tennis balls)…

• How would the cartoon get more electrons to fly of the metal?

The Photoelectric Effect:

The Photoelectric Effect:

SLOPE=PLANK’S

CONSTANT

The Photoelectric Effect:

Shows that light behaves

like a particle

Is light a wave or is light a particle!?!

AIM: How do we describe a particle of light? • Do Now: which experiment shows light is a wave

and which experiment shows light is a particle?

What do we call a particle of light?

A PHOTON!!• A photon is a small ‘packet’ of light

• Any single photon has a fixed, discrete energy. • The intensity of visible light can be increased or

decreased only by changing the number of photons present.

• The same rules hold true for all electromagnetic waves outside the visible range.

Discrete Energy?? • Discrete energy is like money, you can only have integer multiples of

a minimum amount.– For money, what is this minimum amount?

• A photon can only carry integer numbers of a minimum energy– This minimum energy is denoted by Planck’s constant

• h= 6.63x10-34Js• Planck’s constant is modern physics’ version of the

penny• The energy of a photon is determined by its frequency (and

wavelength)

hfE hc

E

UnitsA joule is a large unit of energy. When you are talking about

small electron, we use an electron volt instead

1eV=1.6x10-19JEx1. A photon has 3.5eV of energy. How many Joules of

energy is that?

Ex2. A photon has 4.8x10-19J of energy, how many electron volts is that?

Examples1. A photon of light has a frequency of 2.5x1014 Hz.

- What is the energy of that photon in Joules?- What is the energy of that photon in eV?

2. A photon has a wavelength of 575nm- What is the frequency of that photon?- What is the color of the photon?- What is the energy of that photon in Joules?- What is the energy of the photon in eV?

AIM: How has our understanding of the atom changed over the years?

DO NOW: Draw AND label a diagram of an atom.Quiz…?

Atomic Structure

A brief History

Democritus- Greek Philosopher~300BC

• The word atom means smallest piece. Something that can not be divided.

• Atoms are made of the same ‘stuff’ but differ in size and shape

• Atoms are in constant motion• Atoms can combine to form different types of

matter

John DaltonLate 1700s

• All elements are made up of atoms• Atoms of the same element are all the same

but differ from atoms of different elements.• Atoms can group together to form molecules• Chemical reactions are changes in

combinations of atoms, not changes in the individual atoms themselves.

JJ Thomsonlate 1800s

• Measured the charge/mass ratio of an electron.

• Determined that an electron had a negative charge

• Could NOT determine the actual mass or charge of an electron.

• Plum pudding model of the atom

Negative ‘plums’

Positive Goop

Rutherford-Geiger-Marsden1911

• Gold foil scattering experiment– Fired positively charged alpha particles (2 protons and 2

neutrons) at a thin foil of gold.– Most alpha particles traveled straight through• Most of an atom is empty space

– One day, one scattered at a wide angle as if it hit something massive and dense.• Holds most of the mass of an atom• Must be positively charged

– This massive and dense thing was called the nucleus.– An atom’s diameter is MUCH larger than that of the

nucleus.

Rutherford Scattering Setup

Most particles go straight through. A few scatter and light up the screen at other angles.

Bohr (Orbital) Model• Electrons orbit around

a central nucleus– The electron orbitals

have definite (discrete) energy levels.

– Electrons can not exist between energy levels.• Similar to the fact that

you can not stand between rungs of a ladder.

Bohr (Orbital) Model• Ground states– Electrons want to fill the

lowest possible levels so that the atom stays stable.

• Excited states– Electrons can ‘jump’ up

energy levels only if the correct amount of energy is absorbed by the electron.

– This amount of energy is determined by the energy difference in the atom’s levels.

Cloud Model• The electrons do not exist in

definite orbits around the nucleus like suggested, they live in clouds where there is a high probability that they will be in a certain place. Other regens have a low probability.

Bohr: The Hydrogen Atom1. What is the energy of the n=3

energy level in the hydrogen atom?

a. What is this energy in Joules?

2. What is the energy difference between the n=1 and n=4 energy levels?

Hydrogen Absorption SpectrumWhen light is incident on a hydrogen atom, it can absorb

the photons with the correct amount of energy that allow the electrons in the atom to ‘jump’ to their excited states. An absorption spectrum is the rainbow of colors with the colors matching the correct energy jumps missing.

1. Pick one of the missing colors2. Determine a possible frequency of that color using

the RTs3. Calculate the energy a photon of that color4. Convert that photon’s energy into eVs.5. Using your RTs decide which energy level

transition could be caused by that photon.

Hydrogen Absorption Spectrum

Hydrogen Emission Spectrum

Once an electron has reached the excited state by absorbing the correct amount of energy. The electron will stay there for a moment then return back down to the ground state. When the electron falls back to the ground state, it emits a photon with an energy equal to the energy difference between the level it came from and the level it went to.

Hydrogen Emission Spectrum

1. Pick a different color than before.2. Determine a possible frequency of that color using

the RTs3. Calculate the energy a photon of that color4. Convert that photon’s energy into eVs.5. Using your RTs decide which energy level

transition could be caused by that photon.

All Hydrogen Emissions

I was doing some particle physics research and I discovered 7 new elements. I knew that each element was different because

_______________________________________.

I was able to draw diagrams of each element’s energy levels to scale, and I was able to name each element’s spectrum, but I was not able

to match the element’s energy level diagram to its corresponding spectrum.

Your goal is to use the scaled drawing to figure out the letter of that element based on the atomic spectra pictured.

a. Show all calculations in an organized manor including formulas and units.

b. Choose a fourth color for the spectrum and add the corresponding fourth energy level to the element’s diagram. (it must be drawn to

scale)

F

A B C D

E G

How do LASERS work?

What is Coherent Light?NOT coherent-Many frequencies (colors)- Different phases

NOT coherent-Same frequencies (colors)- Different phases

Coherent-Same frequencies (colors)- Same phases

The Standard Model of Particle Physics

Things smaller than protons and neutrons

QuarksThe building blocks of Protons and Neutrons

• A proton is made up of two up quarks and a down quark (uud)

• A neutron is made up of two downs and an up (udd)

Antiparticles

• Antiparticles have the same mass as their particle ‘buddies’ just the opposite charge and quark make up.

• If a particle and an antiparticle collide, they annihilate each other and all the mass is converted into energy.1. What is the quark make up of an antiproton?

2. If a neutron and antineutron collide and annihilate each other, how much energy is released in Joules?

Classification of MatterProtons and

neutrons have 3 quarks, so they are

Baryons!

Mesons are any particle with a

quark and antiquark

Leptons

• Electrons are leptons!

Make up your own particles

1. A baryon with a +1 charge2. A baryon with a -2 charge3. A meson with a neutral charge4. A meson with a -1 charge5. A baryon with a +3 charge6. A meson with a -2 charge

Nuclear Physics

Subatomic Forces and Structures

Zooming in on the World Around Us

Macroscopically- really big•Gravity–holds all objects with mass together (from stars to dust)•Electromagnetic Force–Holds the (negatively charged) electrons in orbit around the

(positively charged) nucleus of an atom•Strong Force– Holds all the positively charged protons and neutral neutrons

together in the nucleus•Weak Force–Holds all the quarks together in a proton and neutron

Microscopically – really small

Force Name Relative Strength

Force Range Force acts on

Strong Force

Weak Force

Electro-magnetic Force

Gravitational Force

1

2

3

4

Very ShortVery Short

Infinite

Infinite

Nucleons

quarks

Charges

Mass

Creating Nuclear EnergyMass or energy can never be created or destroyed, only

converted from one to the other!

Units of Mass• Kilograms: for larger objects• AMU (atomic mass units) sometime just u for short: The

mass of very small things– Proton: 1.007u– Neutron: 1.009u

• To convert mass in kilograms to energy in Joules used E=mc2 • To convert mass in u into energy in eV use the conversion in

your Reference tables (1u=931MeV) **1MeV=1,000,000eV**

2mcE

Creating Nuclear EnergyMass or energy can never be created or destroyed, only

converted from one to the other!

Fusion• Two smaller elements

(anything below iron) fuse together to create a larger element.

• This is favored by nature because this process releases energy.

Fission • One larger element

(anything above Iron) split apart to create two smaller elements.

• This is favored by nature because this process also releases energy

2mcE

Fusion up Close• For light elements (up to Iron), fusing two elements together

creates a larger element and energy.• This energy comes from the ‘missing’ mass.– The larger element has a smaller mass then the total mass of the

parts that make it up. – The difference in mass is converted into released energy.

• This only happens in the sun and starts

Fission up Close• An incident neutron causes a large unstable

element to split into smaller elements.• When the element splits, some of the energy used

to hold the large nucleus is released.• This happens in nuclear reactors around the world.

E=mc2

1. Which particle would generate the greatest amount of energy if its entire mass were converted into energy? EXPLAIN– electron – proton – alpha particle – Neutron

2. If a proton was completely turned into energy, how much energy would be released?

IsotopesSame element (same number of protons)

Different masses (different numbers of neutrons)

- Can be separated by using a mass spectrometer

Mass (u)

Mass Defect• The mass of the individual protons and neutrons that make up an

element is larger than the actual mass of the element. • This ‘mass defect’ is converted into the energy needed to hold the

nucleus together.

If the actual mass of the Lithium atom is 6.941u, -What is the mass defect in u-What is the binding energy in MeV?-What is the binding energy in Joules?

Knowledge Test

• How much energy would be released if an average high school student was vaporized into energy?

• A hydrogen bomb produces energy when a diatomic hydrogen element (2p) is split into two individual hydrogen atoms. A diatomic hydrogen nucleus has a mass of 2.009u and proton has a mass of 1.00794u. How much energy is produced when one single atom is split?– In MeV– In Joules