What Is Light?

A History

So What Is Light? Answer: Part of the Electromagnetic Spectrum

The Earth Is Bombarded By EM Radiation

In the Beginning Two Theories of Light

Emission Theory

• Light rays originate from within the eye.

• Illuminate an object and are reflected back.

Intromission Theory • Visual perception comes form

something representative of the object being viewed. – Something travels from the

object into the eye.

The Emission Theory

• 5th Century BCE: Empedocles – Aphrodite made the

human eye out of the 4 elements: air, water, earth, fire.

– Lit a fire in the eye. • Evidence: the eyes of an

animal at night. – Resulting light interacted

with rays from the Sun to illuminate an object.

• Emission theory embraced by: – Plato (400 BCE) – Ptolemy (2nd Century CE)

• And Euclid

Euclid’s Book: Optics Embraced the Emission Theory

* 7 Self-Evident Axioms: – Lines (or visual rays) can be drawn in a

straight line to the object. – Those lines falling upon an object form a

cone. – Those things upon which the lines fall are

seen. – Those things seen under a larger angle

appear larger. – Those things seen by a higher ray, appear

higher. – Right and left rays appear right and left. – Things seen within several angles appear

clearer.

• 36 Propositions

• Euclid relates the apparent size of an object to its distance from the eye and investigates the apparent shapes of cylinders and cones when viewed from different angles.

Euclid in Raphael’s “School of Athens” (Fresco)

But Aristotle Embraced the Intromission Theory

• Visual perception comes from something representative of the object (later established to be rays of light reflected from it) entering the eyes.

• Lucretius (55 BCE): The light and hear of the Sun are “composed of minute atoms which are shoved off and lose no time in crossing space”.

Plato and Aristotle From Raphael’s “School of Athens”

Enter Alhazen: 965 to 1040 CE

Father Of Modern Optics

• Born in Persia, died in Alexandria. • Wrote 200 books.

– Only 55 survive. • Lived during the high point in

Islamic scholarship.

• Alhazen admired Aristotle and the early Greeks. – But embraced the scientific

method.

* Alhazen’s ambition got him into trouble.

• To curry favor with Caliph, he said he could control Nile flooding.

– Dam at Aswan. • The Caliph hired him to do that ---and to

get results immediately. – An impossible expectation.

• To save himself from death, Alhazen feigned madness.

– Was able to outlive the Caliph. • When he could risk being sane, he made

major contributions in optics. • Little further done in optics for 600 years.

“The Treasury of Optics” Alhazen’s Major Treatise: 1000 CE

Front page of “The Treasury of Optics”: Archimedes sets fire to Roman ships using parabolic mirrors.

• Observations: • If you look at the Sun, your eyes

hurt. Bright objects hurt the eye. • The brighter the object the

brighter it appears to be. • An experiment with 5 lanterns

outside a hole in a wall: • Five light spots on inside wall. • In reverse order.

• Conclusions: – Light is made up of a stream of

small particles that are reflected to the eye by an object.

– The particles go in straight lines. – One beam of light does not

interfere with another.

Alhazen’s Contributions To Understanding Light

• Showed light could be separated into its constituent colors.

• Explained the pin-hole camera.

• First to describe eye as an optical instrument. – Explained how a lens could

focus light. • Studied parabolic and

spherical mirrors.

Developed Laws of Reflection and Refraction

Isaac Newton (1642 to 1727) Major Advances In Understanding the Nature of Light

Isaac Newton At Age 46 1689 Portrait by Kneller

• Possibly the greatest scientist in history.

• Born 3 months after death of father. – Came to hate mother and

stepfather. • Angered by criticism.

– Harsh toward enemies. – Harbored resentment.

• Never married. • Major discoveries made when

25 to 27 years old.

Newton’s Theology • An Arian.

• Did not believe in the Trinity.. • Did not believe Jesus was the Son

of God.

Descendent of Newton’s Apple Tree Cambridge University

* Newton believed he was chosen (by God?) to understand Biblical

scripture. • Hidden messages in Bible. • World will last at least until 2060.

– Then “Paradise” on Earth. * Believed in alchemy. * Newton: “Gravity explains the motion of the planets but it cannot explain who set the planets in motion. God governs all things and knows all that is or can be done.”

Newton’s Achievements • Discovered 3 Laws of Motion • Developed the Law of Gravity. • Asserted that the laws of physics applied throughout the Universe. • He (with others) invented calculus • He made many discoveries in pure mathematics.

• Published “The Nature of Light” in 1704.

The Nature of Light [Title Page from “Opticks”: Published 1704]

• Light composed of transparent “corpuscles”. – Traveled in a straight line.

• “Color” was a property of light. – Not affected by reflection or

refraction. • The “color” of an object

resulted from an interaction with the colors within light. – Colors arise from selective

absorption, transmission and reflection.

Prism Experiments

• Light can be broken into its component parts.

• And recombined to form white light.

Prism Experiment 3

• A light component will keep its color when passing through another prism.

• Once separated, a color could not be further separated.

Newton vs. The Scientific Establishment

• Prior to Newton, colors were thought to be “modified” forms of white light. – “Color” a mixture of light and dark. – The prism colored the light.

• Newton did not have close relationships with many other European scientists. – Some criticized his work on optics.

• Newton did not accept criticism lightly. – Delayed publishing “Opticks” until his critics were, mostly,

dead. • Newton believed there was an “ether” in which light

traveled.

Newton’s Color Wheel

• For artists the “primary” colors are: – Red – Yellow – Blue

• Mix to get all other colors.

• The primary colors that give white light are red, green and blue.

Reflecting Telescope • Refractory telescopes: chromatic

aberration problems. – Lens acts like prism. – Newton recognized this.

• Problem solved with a reflecting telescope.

– Invented by a Scot (Gregory), but improved by Newton.

Newton’s Contributions To Understanding Light

• Showed that “color” was a property of light. – Not affected by reflection or refraction.

• The color of an object resulted from its interaction with the colors within light.

• Light was corpuscular – Velocity changes when the corpuscle enters a new

medium. • “Color wheels” • Invented the reflecting telescope

A Short Pause Concept of A Field

• How do you explain “action at a distance”? • Newton had a problem with gravity: how can

a force be exerted “invisibly” over long distances? – This played into Newton’s ideas of the occult.

• The solution: “A Field”.

Fields: Electric, Magnetic, Gravitational

• An Electric Field: – Force exerted on a charged

particle. – Like particles repel,

oppositely charged particles attract.

• A Magnetic Field: – The magnetic “field” shown

by iron filings. – Some materials are naturally

magnetic.

The Earth’s Magnetic Field • Magnetic pole 11o from spin axis.

– About 1000 miles from geometric pole in Arctic and 1600 miles from geometric pole in Antarctic.

• Aurora Borealis a result of solar particles interacting with Earth’s magnetic field.

• The 2011 earthquake off Japan shifted the location of the north magnetic pole.

So: What Is A Field?

• A “field” is a way of describing force at a distance. – Invisible “springs” with

specific properties.

• From the point of origin of the field, the force at a distance r varies as 1/r2

• The concept of a field is important when we talk about light and electromagnetism

James Clerk Maxwell: 1831 to 1879 Light and Electromagnetism

One of history’s greatest mathematicians and physicists.

• Entered Edinburgh University at age 16.

• At age 19, was an accomplished mathematician.

• At age 25, became professor at the University of Aberdeen.

• At age 34, completed his major work on electromagnetism: “A Dynamical Theory of the Electromagnetic Field” (1864). – This is considered one of the

greatest advances in the history of physics.

Maxwell’s Achievements: Made Sense Out of the Work of Others

• Hans Oersted (Dutch physicist): an electric current creates a magnetic field.

• Michael Faraday (English physicist): a changing magnetic field creates an electric current

• Developed the mathematics that explained electromagnetic (light) phenomena.

Hans Oersted: 1777 to 1851 Oersted’s Experiment: 1820

Dr. David Stern NASA Goddard Space Flight Center

• Goal of experiment: to show that a wire would warm up if a current ran through it.

• By accident, Oersted noticed that a nearby magnetic needle would point at right angles to the wire when current flowed.

• The magnetic force (field) was circular about the conducting wire.

• CONCLUSION: A moving electric charge created a magnetic field.

• If the electric current were in a coil, the magnetic field would be inside the coil.

Electromagnet

• Electricity flowing in a coil of wire creates a magnetic field inside the coil.

• An iron bar inside the coil becomes a magnet.

Michael Faraday: 1791 to 1857

Faraday’s Experiment in 1831 • Two independent coils around an

iron ring. • “A” connected to a battery. • Copper wires attached to “B” go

to a magnetic needle. • Turn current on to “A”.

– Iron ring becomes a magnet (Oersted).

• Magnetic needle briefly oscillates, then settles back to original position.

• Turn off current to “A”: – Same thing happens.

• Magnetized ring induced a current in “B”. Conclusion: changing magnetic field

creates an electric current

Faraday’s Test Coil And Laboratory

Photograph of Test Coil

Painting Harriet Jane Moore

1950s

Electric Waves and Magnetic Waves Travel Together

• Moving electrons create a magnetic field.

• A changing magnetic field creates an electric field.

• They travel at right angles at the speed of light.

Maxwell’s Contributions To Understanding of Light

• Developed the physics and mathematics that describe how: – A changing electric field produces a

magnetic field. – A changing magnetic field

produces an electric field. • Together they constitute

electromagnetic radiation – waves.

• The magnetic and electric components of that wave are at right angles to one-another.

• What is it that the waves “wave” in? – An ether.

• Light is a an EM wave. • EM waves travel at “c”, the speed

of light (in a vacuum).

Are Electromagnetic Fields Harmful To Your Health?

• There appears to be a small increased risk of leukemia from exposure to very low frequency EM radiation. – The scientific evidence is weak. – No increase has been seen in experimental

animals.

• There appears to be no risk at environmental levels.

Albert Michelson: 1852 to 1931 The Speed of Light In 1887

• Polish born physicist • U.S. Naval Academy in 1869.

– Professor of physics at Academy in 1877.

• Conducted first experiments on the speed of light while at the Academy.

• Set out to prove there was an “ether” filling the Universe

• Michelson-Morley experiments in 1887.

• Nobel Prize for Physics in 2007. – First American so honored

Michelson, Einstein and Morley [California Institute of Technology: 1931]

The Speed of Light Through the Centuries

• Light faster than thunder. • Empedocles in 450 BCE Ancient Greece: finite. • Aristotle, 4th c. BCE: infinite. • Alhazen, 11th c. CE: finite. • Kepler, 1609: infinite • Galileo, 1632: between two people on distant hills:

– Between 10,000 km/hr. and infinity. • Ole Roemer, Danish astronomer (17th century):

– Analyzed data on eclipses of Jupiter’s moon, Io: • Deduced speed: 190,000 km/sec.

Roemer’s Measurement of the Speed of Light Jupiter’s Eclipses of the Moon Io

E1 and E2 are positions of the Earth relative to the Sun (S). J1 and J2 and corresponding positions of Jupiter when Jupiter eclipses Io.

• Roemer measured the speed of light by timing Io’s (Jupiter’s moon) orbit about Jupiter.

• When the Earth was at E2, the light from the Jupiter system had to travel an extra distance.

• Roemer could then calculate the speed of light.

• 190,000 km/sec.

The Speed of Light Through the Centuries Continued

• Michelson at age 27 at U.S. Naval Academy (1878): – Speed of light is 299,910 (+/- 50) km/sec.

• Current Value: 299.792.458 km/sec. – The length of a meter is defined in this way.

The Concept of Ether [Space “matter”]

• Postulated by the Aristotle: – Fills every point in space.

• Newton: endorsed the notion of an “ether”. – Speed of light relative to ether.

• Maxwell: equations showed EM waves traveled at “c” in a vacuum. – If light is a wave, what does it wave in? – Ether postulated to have properties consistent with

Maxwell’s equations. • Magical properties

Maxwell and the Concept of Ether

• “Whatever difficulties we may have in forming a consistent idea of the constitution of the ether, there can be no doubt that the interplanetary and interstellar spaces are not empty but are occupied by a material substance or body which is certainly the largest, and probably the most uniform body of which we have any knowledge.”

Michelson’s Strategy to Measure the Speed of Light The Swimmer/River Analogy

• Think of the river as an ether flow; The swimmer as a ray of light. • Swimmer: 5 ft./sec. River: 3 ft./sec. Width: 100 ft. • Faster to go across the river twice than to go up river 100 ft. and back 100 ft. • Hence: If there is an ether, the time it takes for a ray of light to go someplace

should depend on the ray direction relative to the ether flow.

Michelson’s Strategy Use the Earth’s Movement

The Earth’s movement around the Sun and the Sun’s movement in space allow light measurements to be made against, with and across the ether.

Experimental Concept The Michelson-Morley Experiment

• Work with monochromatic light. • Split beam into perpendicular

beams. • One beam against the Ether. • One beam across the Ether.

• Reflect the beam back and recombine into one beam. • If they arrive back at different

times, interference. • Expectation: the “cross wind”

time will be less than the “into the wind” time.

• The equipment floated on a pool of Mercury for stability. • But could also be rotated.

Michelson’s Results

• Measured the speed of light. • The speed of light was the same no matter

when or how the experiment was run. • Proved that an “ether” did not exist. • Signaled the beginning of a second scientific

revolution: relativity and quantum mechanics.

Light Is Quantized Max Planck: 1858 to 1947

• German physicist. • “Founder of Quantum Theory”

• Professor of Physics at age 27. • Nobel Prize in 1918 • Personal life: Age 20

• 1st wife died. • 2 daughters died. • Son killed in WW I. • 2nd son killed by Nazis.

• Involved in Hitler assassination attempt

Age 60

Question Addressed By Planck

How does a hot black body cool itself?

Blackbody: • Absorbs all radiation

incident on it. • Radiates energy as

necessary for equilibrium. • Body does not need to

look black to be, essentially, a “black body”.

• The Earth’s surface behaves approximately as a black body.

The Sun and the Earth Blackbody Radiation

Radiation from Earth is of photons of much lower energy than the photons it gets from the Sun.

Lyndon State College, Vermont

• Input to Earth: Relatively short wave length from solar radiation (high temperature)

• Output From Earth: long wave length (low temperature)

Planck’s Law: 1900 Describes How A “Black Body” Cools Off

Black body radiation curves showing peak wavelengths at various temperatures

• To obtain agreement with experiment, Planck had to assume energy was emitted in quanta.

• E = h√ = h/λ –E = energy of quanta. –h = Planck’s constant. –√ = frequency –λ = wave length

• He didn’t appreciate what this physically involved. – But it gave the right answer.

Planck’s Contribution to Understanding Light

• Planck showed that to calculate how bodies cooled off one had to assume that the emitted radiation was in the form of packets of energy. – Quanta not waves.

• Einstein discovered the physical basis for the assumption in 1905.

Proof That Light Comes in Packets Albert Einstein: 1879 to 1955

Einstein at age 25

• Family were non-observant Jews. • He was education initially in a Catholic

elementary school.

• In an out of several schools, Einstein entered Zurich Polytechnic in 1896.

• Graduated in 1900 in physics and math with a teaching diploma.

• Could not find a teaching position.

• Bern Patent Office: 1902 • While at the patent office, he

continued education. • Ph.D. in 1905.

1905: A Year To Remember [Einstein’s: Age 26]

• Special Theory of Relativity. – Newtonian physics not compatible with Maxwell’s equations.

• Equivalence of Mass and Energy – E = mc2

• Brownian Motion

• And in 1916: The General Theory of Relativity. ________________________________________ • The Photoelectric Effect:

– Light travels in quanta. – Received Nobel Prize in Physics in 1921.

The Photoelectric Effect

• Light travels in packets:

quanta. • For low energy quanta, no

electrons emitted. • Above an energy threshold,

electrons emitted – Higher energy, more kinetic

energy of electron. – More intense beam, more

electrons, same kinetic energy.

• Photoelectric materials: cesium, rubidium, antimony

Einstein: The 1921 Nobel Prize

• Einstein: “According to the assumption considered here, in the propagation of a light ray emitted from a point source, the energy is not distributed continuously over ever-increasing volumes of space but consists of a finite number of energy quanta localized at points of space that move without dividing and can be absorbed or generated only as complete units.”

• This discovery gave legitimacy to the assumptions Planck made to understand blackbody radiation. • Einstein received the Nobel Prize in 1921 for his discovery of the photoelectric effect.

Einstein’s Contribution to Understanding Light

• Einstein built on Planck’s work in 1901 on black body radiation. – Energy (light) is emitted as packets of energy.

• He proved that light traveled as packets of energy. – As quanta.

• Visible example: the photoelectric effect. – Photovoltaic cell.

A Brief Aside Subatomic Particles Sometimes Behave Like Waves

Louis de Broglie (1885 to 1962) (7th duc de Broglie) 1929 Picture

• In 1924, Louis de Broglie proposed that subatomic particles exhibit wave-like properties. – Part of his Ph.D. thesis: “Research

On Quantum Theory”. – His predictions were

experimentally confirmed in 1927 with electrons.

• He received the Nobel Prize for Physics in 1929 for this work.

• De Broglie Hypothesis: “Any moving particle or object had an associated wave”. – The wave length of a large object is

so small it cannot be detected.

De Broglie’s Prediction • Particles interact with matter as if the

particles were waves: – λ = h/mv – λ = wavelength associated with

particle = de Broglie Wavelength – h = Planck’s constant – m = mass of particle – v = velocity of particle – mv = momentum of particle

1927 “Accidental” Experiment At Bell Labs Electrons Scatter Off Crystals

Clinton Davisson and Lester Germer (1927)

• Davisson and Germer at Bell Labs discovered electron waves by accident .

• Scattered electrons off a nickel surface trying to improve vacuum tubes.

• Lab accident damaged nickel target. • Subsequent heat treatment of target

caused nickel to crystalize. • Subsequent scattering experiments

with electrons showed interference patterns.

• Reflected electrons show constructive and negative interference as if they were waves.

The Two-Slit Experiment

One of History’s Most Famous Experiments

Light As a Wave • Each slit acts as an individual

source of new light waves.

• Light waves from the two slit sources interfere to form lines of light.

• Classical wave theory gives the right answer.

Light As “Quanta” Quanta (Packages) = Baseballs

• Experiment results not consistent with the notion of quanta = baseballs.

• Query? If light is a “package” why doesn’t it act like a baseball?

• Answer: In this experiment, light must be treated as a wave.

Classical expectation: two light strips on screen

The Double Slit Experiment With Electrons

Reality: interference lines appear. Exactly like waves.

Classical expectation: electrons behave like baseballs.

The Velocity of Light a Constant?

• Einstein’s Special Theory of Relativity: the

velocity of light is a constant. – Independent of the velocity of the source or

observer. – Nothing can move faster than the speed of light.

Einstein’s Thought Experiment

• Hold a mirror in front of your face. • Travel forward at the speed of light. • Will light move forward and be reflected back by

the mirror? – If no, there must be an “ether” filling space. – If yes, there can be no ether.

• Einstein concluded: – There was no ether. – The velocity of light must be a constant independent

of the motion of the source.

So: What Do We Know?

• Light is a form of electromagnetic radiation. • Light travels as a wave.

– A light “wave” is a combination of an electric wave and a magnetic wave.

• But sometimes, light travels as a packet of energy. • Sub atomic particles travel as particles. • But sometimes sub atomic particles travel as waves. • There is no ether. • The velocity of light is a constant, independent of the

motion of the source or observer.