TheThe NatureNature ofof LightLight
Prof.Prof. StephenStephen SekulaSekula SupplementarySupplementary MaterialMaterial forfor PHY1308PHY1308 (General(General PhysicsPhysics –– ElectricityElectricity andand Magnetism)Magnetism) Class Announcements
● Homework 9 assigned today
● Second-to-last homework!!! ● Assignments for Next Class:
● Read 33-5 – 33-6 ● Watch video: – http://youtu.be/nOS0TGeuLoc ● Team Meetings
● Team leaders have my proposed days and time ranges and should be working with you to find a 30-minute slot that works as optimally as possible. First-come, first-served. Stephen J. Sekula - SMU 2 TheThe NatureNature ofof LightLight
Prof.Prof. StephenStephen SekulaSekula SupplementarySupplementary MaterialMaterial forfor PHY1308PHY1308 (General(General PhysicsPhysics –– ElectricityElectricity andand Magnetism)Magnetism) André Mouraux - Light bulbs Dennis Jarvis - DSC_4584 - Peggy's Lights Up Jo Schmaltz - Hong Kong Skyline WHAT IS LIGHT? Galileo Galilei 1564-1642
Considered the first “modern scientist,” Galileo combined mathematics with observation and argued that this was the way to understand the natural world.
He perfected the telescope and observed that the earth could not be the center of the universe. He was tried and convicted by the Inquisition for heresy and was placed under house arrest for the remainder of his life. He died the same year Isaac Newton was born. Speed of Sound
Make measurements of the round trip that sound takes from Dallas Hall to points 100m and 200m south of Dallas Hall.
(Don't get shut down by the cops) “If not instantaneous, it is extraordinarily rapid” – Galileo Galilei Speed of Light Jupiter
Io Discovered by Galileo in 1610
Earth
Ole Roemer Roemer estimated the speed of light to 8 First to measure speed of light be 2.2x10 m/s by observing Earth- to be finite (1676) seasonal variations in Io's orbit. ENTER ELECTRICITY AND MAGNETISM... Michael Faraday Johann Carl Friedrich Andre-Marie Ampere (1791--1867) Gauss (1775--1836) Brilliant experimental (1777--1855) French physicist and scientist, perhaps one of the “The Prince of Mathematics,” mathematician who greatest that ever lived. Gauss contributed to many established the relationship Made many discoveries, fields. He established how field between enclosed current including magnetic flux relates to enclosed charge, and encircling magnetic induction. and his discovery was the more field. His discovery was the fundamental version of more fundamental version Coulomb's Law. of the Biot-Savart Law. The 4 Laws of electricity and magnetism
d ΦB d ΦB Faraday's Law – changing magnetic magnetic flux flux induces ε=E⃗-⋅d ⃗s=- electricinduces potentialan electric difference field (voltage) ∮ dt dt
⃗ μ i d ⃗L×r^ TheAmpere's Biot-Savart Law – Lawelectric – electric current current is the source is the ∮ B⋅d ⃗s0=μ0 i B⃗ =∫ sourceof magnetic of magnetic field field r 2
⃗ ⃗ ⃗1 1 q Coulomb'sGauss's Law Law for Electric– electric Fields charge – electricis the source of ∫E=E∫⋅d A= q2 r̂ electriccharge fieldis the source of electric field 4 π ϵ0ϵ0r
B⃗⋅d A⃗=0 Gauss's Law for Magnetic Fields – there are no ∫ individual north and south poles (magnetic charges) The 4 Laws of electricity and magnetism - “Free Space”
d ΦB Faraday's Law – changing magnetic flux can E⃗⋅d ⃗s=- still induce an electric field ∮ dt
B⃗⋅d ⃗s=0 Ampere's Law – if there are no currents, there ∮ are no magnetic fields.
E⃗⋅d ⃗A=0 Gauss's Law for Electric Fields – if there are no ∫ charges, there are no electric fields.
B⃗⋅d A⃗=0 Gauss's Law for Magnetic Fields – there are no ∫ individual north and south poles (magnetic charges) James Clerk Maxwell 1831-1879
Brilliant scientist working in Britain. ● United electricity and magnetism into a single “force” ● Developed a theory of large numbers of particles ● Made the first true color photograph
Published in 1864 “A Dynamical Theory of the Electromagnetic Field,” which was seen by Faraday before his death in 1867. The 4 Laws of electricity and magnetism - “Free Space”
d ΦB Faraday's Law – changing magnetic flux induces E⃗⋅d ⃗s=- an electric field ∮ dt
d Φ Ampere's Law – a time-varying electric flux ⃗ E induces a magnetic field – A ∮ B⋅d ⃗s=μ0 ϵ0 dt PREDICTION.
E⃗⋅d ⃗A=0 Gauss's Law for Electric Fields – electric charge ∫ is the source of electric field
B⃗⋅d A⃗=0 Gauss's Law for Magnetic Fields – there are no ∫ individual north and south poles (magnetic charges) The solutions to Maxwell's Equations
⃗ ^ E (x ,t)=E max sin ( k x−ωt ) j ⃗ ̂ B(x ,t)=Bmax sin ( kx−ω t ) k
where k and ω are constants.
What do these equations describe??? Waves λ Wavelength ) A (
Sine e d u t i l p m A
x Square u x ,t=Asin 2 −2 f t
Triangle
Sawtooth Electromagnetic waves
"Electromagneticwave3D" by Lookang many thanks to Fu-Kwun Hwang and author of Easy Java Simulation = Francisco Esquembre - Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:Electromagneticwave3 D.gif#/media/File:Electromagneticwave3D.gif Electromagnetic waves At what speed do electro-magnetic waves travel?
Answer from Maxwell's Equations: 1 v = = . . . EM Wave ϵ μ √ 0 0 Electromagnetic waves At what speed do electro-magnetic waves travel?
Answer from Maxwell's Equations:
1 8 v = =3.0×10 m/s EM Wave ϵ μ √ 0 0 Heinrich Hertz Guglielmo Marconi Robert Hyer (1857-1894) (1874-1937) (1860-1929) First to satisfactorily Italian inventor who Physicist, Founder and First demonstrate the existence of developed the radio President of SMU EM waves telegraph system First American to (first demonstrated in communicate using EM 1894) waves (1894) Transmitter Receiver
A detailed schematic of Hertz's experiment to test the prediction by Maxwell that electromagnetic waves travel at the speed of light, like light waves. This device generated Hertz's 1893 ~100MHz EM waves publication. Robert Hyer demonstrates the use of x-rays to faculty at Southwestern University
Original credit for the photo of Robert Hyer demonstrating the use of x-rays to faculty members at Southwestern University: Special Collections, A. Frank Smith, Jr. Library Center, Southwestern University, Georgetown, Texas.
Question
If water waves travel in water . . . and sound waves travel in air . . . in what medium does light travel? ANSWER from Maxwell's Equations: NONE???
d ΦB Faraday's Law – changing magnetic flux induces E⃗⋅d ⃗s=- electric field ∮ dt
d Φ Ampere's and Maxwell's Law – changing electric B⃗⋅d ⃗s=μ ϵ E flux is the source of magnetic field ∮ 0 0 dt
E⃗⋅d ⃗A=0 Gauss's Law for Electric Fields – electric charge ∫ is the source of electric field
B⃗⋅d A⃗=0 Gauss's Law for Magnetic Fields – there are no ∫ single magnetic charges
Schematic of the best version of Michelson and Morley's famous experiment.
The physics behind the Michelson-Morley Experiment, explaining how light traveling parallel or perpendicular to the “luminiferous aether” would be slowed/sped by the motion Example interference pattern and cause an interference pattern to occur. Albert Einstein (1879-1955)
In 1905, published four papers on atomic theory, the nature of light, and the re-interpretation of space and time based on the theory of electromagnetism (“relativity”). Einstein Reimagined the Universe
● Space and time
● Newton, Galileo, etc. - assumed space and time were a fixed frame of reference in which all events happen; observers might disagree on spatial measurements (e.g. displacements) but they all agree that time intervals are the same. ● Einstein: all observers agree on the events themselves, but disagree on the reasons they happen; space AND time are different for observers moving at different speeds relative to one another or to an external reference point. ● Light has BOTH wave and particle properties
● The laws of electromagnetism describe light as a wave ● Einstein proposed that under certain conditions, light may have “particle” behavior as well (the photoelectric effect, discovered by Hertz) ● These concepts launched twin revolutions: Relativity (A General Theory of Space and Time) and Quantum Physics (A General Theory of Matter and Forces) THE PATH TO MODERN PHYSICS From the 1800s to the 1900s
● We have a complete set of laws for:
● Mechanics: the theory of space, time, motion, and energy. Unified the heavens and the Earth in a single explanatory framework. ● Electricity and Magnetism: a “theory of light,” it explains all electric and magnetic phenomena and predicts the unification of these seemingly distinct forces through light itself. ● But there were observations at the beginning of the 1900s that defied explanation by these laws. A Set of Quirky Observations
● Atomic Spectra
● The light emitted from atomic elements when they were excited by an electric current displayed strange patterns with bright bands and dark gaps - “spectra”. Mechanics and E&M predicted that such emissions would be “continuous” - no gaps, no bands. ● Recall that the electron would not be discovered until 1897, and the first models of the atom imagined electrons embedded in a sea of positive charge – a “plum pudding model of the atom” ala J. J. Thomson.
HYDROGEN A Set of Quirky Observations
● UV VISIBLE INFRARED “The Ultraviolet Catastrophe” 14 5000 K ) ¹ ⁻ 12 m n
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² Classical theory (5000 K)
Mechanics applied to the ⁻ m
10 ·
¹ ⁻ r s
phenomenon of “heat” predicted · 8 W k (
e c
that for certain kinds of heated n 6 a i d a r
l 4000 K a
r 4 t
bodies that can absorb all c e p S 2 frequencies of light shone on 3000 K
0 them. Mechanical theory 0 0.5 1 1.5 2 2.5 3 predicted that at equilibrium Wavelength (μm) (energy in = energy out), such bodies would emit infinite power... which, in reality, they do not. A Set of Quirky Observations
● The Photoelectric Effect
● Mechanics and Electromagnetism together predicted that light waves shone on a metal should produce an electric current that depends on the intensity and the energy of the light. ● In reality, it was observed that only ENERGY had an effect; below a certain energy, no amount of intensity would make an electric current flow. A Set of Quirky Observations ● Radioactivity
● Discovered in 1896, it was found that certain atomic elements were unstable and spontaneously emitted energy. This could not be explained by Mechanics or Electromagnetism. Henri Becquerel (1852-1908) Discovered radioactive decay in 1896 while working with materials that glowed in the dark.
Pierre and Marie Curie Demonstrated the complexity of radiation. Questions answered incorrectly by Mechanics and E&M ushered in a new era of discovery in the 1900s - “Modern Physics” Maxwell's Equations Inside!
“THE STANDARD MODEL” Want More? ● COSMOS (2014) – hosted by Astrophysicist and Director of the Hayden Planetarium Neil deGrasse Tyson
● Episode 10 - “The Electric Boy” (discusses the life and discoveries of Michael Faraday and the role of James Clerk Maxwell near the end of Faraday's life). Want More?
● “Einstein's Big Idea,” NOVA (PBS)
● http://www.pbs.org/wgbh/nova/physics/einstein-big-idea.html Want More?
● Check out “The Fabric of the Cosmos” hosted by Columbia University physicist and professor Brian Greene:
● What is Space?
● The Illusion of Time Want More?
● “Einstein's Revealed,” NOVA (PBS)
● Learn about the life of Albert Einstein, his ideas, and his relationships ● “Inside Einstein's Mind” NOVA (PBS)
● Learn about the “thought experiments” that helped lead Einstein into the deep and difficult mathematics needed to understand space, time, energy, and matter. http://www.pbs.org/video/2365615918/ Additional Material How do we know light is a wave?
Photo credit: Paolo Desiati taken at Lake Michigan All observed mechanical waves exhibit certain characteristic behaviors – such as “interference,” the addition/subtraction of waves when they meet How do we know light is a wave?
Mechanical waves “diffract” when they encounter a barrier with an opening in it. The port of Alexandria above shows “single-slit diffraction” of water waves – plane waves incident on the opening spread out after the opening. Two-slit diffraction (right) allows for interference to occur. How do we know light is a wave?
Light diffracts when passed through an opening (left). Light interferes, as when two sources of light are allowed to have their waves overlap (right).