Let there be ...

LightLight

The Nature of LightThe Nature of LightWhat is Light?

By the By the 17th century17th century,,light had been observed to…light had been observed to… 1.1. travel in straight lines travel in straight lines 2.2. reflect reflect 3.3. refract refract 4.4. transmit energy from transmit energy from one place to anotherone place to another

Two theories could explainTwo theories could explainthese phenomena.these phenomena.

The The WAVE THEORYWAVE THEORY,, advocated by advocated by Christian Huygens and and Robert Hooke,, said that light was a said that light was a wave.wave.The The PARTICLEPARTICLE (corpuscular) (corpuscular) THEORYTHEORY, ,

advocated by advocated by Isaac Newton and later by and later by Pierre Laplace, said , said that light was made up of a streamthat light was made up of a stream of tiny particlesof tiny particles called corpuscles.called corpuscles.

The more popular theory was the The more popular theory was the theory theorybecause because ..

particleof the reputation of Isaac Newton.

Newton’s particle theory could easily explain theNewton’s particle theory could easily explain thestraight line travel of light, reflection, and energystraight line travel of light, reflection, and energy

transmission, but had trouble explaining refraction.transmission, but had trouble explaining refraction.

Newton’s explanation of refraction required thatNewton’s explanation of refraction required thatlight must travel light must travel faster in water than in airfaster in water than in air..

Huygen’s wave theory could easily explain reflection,Huygen’s wave theory could easily explain reflection,energy transmission, and refraction, but had difficultyenergy transmission, and refraction, but had difficulty

explaining the straight line travel of light.explaining the straight line travel of light.

The wave theory’s explanation of refractionThe wave theory’s explanation of refractionrequired that light must travelrequired that light must travel

slower in water than in airslower in water than in air..

The debate among the two sides continuedThe debate among the two sides continuedthrough the mid 1800’s.through the mid 1800’s.

18011801 interferenceinterference of light was discovered of light was discovered18161816 - - diffractiondiffraction of light (actually observedof light (actually observed in the 1600’s but not given much significance)in the 1600’s but not given much significance) was explained using interference principleswas explained using interference principles

Neither phenomena could be explainedNeither phenomena could be explainedsatisfactorily by the particle theory.satisfactorily by the particle theory.

The final blow to the particleThe final blow to the particletheory came in theory came in 18501850 when when Jean Foucault discovered thatdiscovered thatlight traveledlight traveled faster in airfaster in airthan in waterthan in water..

It was then widely accepted that light was a wave,It was then widely accepted that light was a wave,but what kind of wavebut what kind of wave??

In 1865, In 1865, James Maxwell developed ideas began by developed ideas began by Michael Faraday into a series of equations thatseries of equations thatproposed the proposed the electromagnetic wave theoryelectromagnetic wave theory. It. Itsaid that light was a type of e/m wave: said that light was a type of e/m wave: a periodica periodicdisturbance involving electric and magnetic forcesdisturbance involving electric and magnetic forces..

In In 18651865, , Heinrich Hertzexperimentally confirmedexperimentally confirmedthe e/m theory.the e/m theory.

Click Click here and here to view simulations of

electromagnetic waves.electromagnetic waves.

Click Click here to explore a tutorial on theproduction of e/m waves.production of e/m waves.

At the end of the century, many physicists felt thatAt the end of the century, many physicists felt thatall the significant laws of physics had been discovered.all the significant laws of physics had been discovered.HertzHertz even stated, “The wave theory of light is, from even stated, “The wave theory of light is, from

the point of view of human beings, a certainty.”the point of view of human beings, a certainty.”

That view was soon to change.That view was soon to change.

Around Around 19001900, , the the photoelectric effect was observed. was observed.

““the emission of electrons by a substancethe emission of electrons by a substancewhen illuminated by e/m radiation”when illuminated by e/m radiation”

Careful study of the photoelectric effectCareful study of the photoelectric effectwas performed by many scientists.was performed by many scientists.

The wave theory could not totally explain theThe wave theory could not totally explain thephotoelectric effect, but a variation of thephotoelectric effect, but a variation of the

old particle theory could!old particle theory could!

Max Planck andand

Albert Einsteinsubsequently proposed thesubsequently proposed the

QUANTUM THEORYQUANTUM THEORY..

The Quantum TheoryThe Quantum TheoryThe transfer of energy betweenThe transfer of energy between

light radiation and matter occurs inlight radiation and matter occurs indiscrete units called discrete units called quantaquanta, the magnitude, the magnitude

of which depends on the frequency of radiation.of which depends on the frequency of radiation.

Although we still commonly characterizeAlthough we still commonly characterizelight as a wave, it is actually light as a wave, it is actually neitherneither a awave nor a particle. It seems to havewave nor a particle. It seems to have

characteristics of both.characteristics of both.

The modern view of the nature ofThe modern view of the nature oflight recognizes the dual character:light recognizes the dual character:

Light is radiant energy transportedLight is radiant energy transportedin photons that are guided alongin photons that are guided along

their path by a wave field.their path by a wave field.

This leads us to theDuality PrincipleDuality Principle:

Light is ...Light is ...• a wave when it acts like a wavea wave when it acts like a wave• a particle when it acts like a particlea particle when it acts like a particle

Visible lightVisible light is that portion of the is that portion of theelectromagnetic spectrum which electromagnetic spectrum which stimulates the retina of thestimulates the retina of thehuman eye.human eye.

Visible spectrumVisible spectrumwavelengths rangewavelengths rangefrom about from about 400 nm 400 nm (violet)(violet) to 760 nm to 760 nm (red)(red)..

Light travels at about Light travels at about 3 x 103 x 1088 m/s m/s throughthroughempty space and slightly slower empty space and slightly slower through air.through air.

Remember that for all waves, Remember that for all waves, v = fv = f..

CCOOLLOORRMaterials may be classified as:Materials may be classified as:

transparenttransparent - readily transmits light;- readily transmits light;can clearly see objects through themcan clearly see objects through them

translucenttranslucent - transmits, but diffuses, light; - transmits, but diffuses, light;cannot see objects clearly through themcannot see objects clearly through them

opaqueopaque - transmits no light; - transmits no light;cannot see through themcannot see through them

WHITE light is composed of all colors.

RedRed, , orangeorange, , yellowyellow, , greengreen, , blueblue, , violetvioletis the order ofis the order of

increasing frequencyincreasing frequencyor or decreasing wavelengthdecreasing wavelength..

Frequencies directly above this spectrum areFrequencies directly above this spectrum are

ultravioletultraviolet..

Frequencies directly below this spectrum areFrequencies directly below this spectrum are

infraredinfrared..

The color of an The color of an opaqueopaque object depends object dependson the on the colorscolors ( (frequenciesfrequencies) of light) of lightincidentincident upon it and on the upon it and on the colorscolors((frequenciesfrequencies) of light ) of light reflectedreflected..

The color of a The color of a transparenttransparent object depends object dependson the on the colorscolors ( (frequenciesfrequencies) of light) of lightincidentincident upon it and on the upon it and on the colorscolors((frequenciesfrequencies) of light ) of light transmittedtransmitted..

Complimentary colorsComplimentary colors are twoare twocolors that combine to form white light.colors that combine to form white light.

RedRed and and cyancyan, , blueblue and and yellowyellow, ,

greengreen and and magentamagentaare pairs of complimentary colors.are pairs of complimentary colors.

Red, blue, and green are calledprimary colors or secondary pigments.

Cyan, yellow, and magenta are calledprimary pigments or secondary colors.

These sites let you simulate mixing These sites let you simulate mixing colors and pigments of light: colors and pigments of light: link1, link2, link3

Learn more about color mixing Learn more about color mixing here.

“ “But I learned that the But I learned that the primary colors are red, primary colors are red, blue, and yellow – not blue, and yellow – not red, bluered, blue, and green.”green.” Read about it here. Read about it here.

POLARIZATIONPOLARIZATION

Only transverse wavesOnly transverse wavesmay become polarized.may become polarized.

Click here, here, here, and here Click here, here, here, and here to explore polarization of light.to explore polarization of light.