Biomedical Engineering Reference
In-Depth Information
the.straight.line.between.the.two,.some.of.the.light.will.“get.around”.the.screen.and.still.illuminate.B..
he.presence.of.a.inite.size.Fresnel.zone.is.strong.evidence.of.a.wave.explanation.to.light.propagation.
If.the.hole's.diameter.is.less.than.or.equal.to.that.of.a.Fresnel.zone,.then.a.diferent.wave.optic.efect.
takes.place,.
difraction
.from.the.hole..If.the.hole.is,.once.again,.in.line.with.the.path.from.A.to.B,.then.
light. is. still. detected. at. B,. but. at. a. lower. intensity. since. some. of. the. possible. contributing. paths. are.
choked.of.
But.now.imagine.that.the.hole.is.placed.of-center.from.the.line.connecting.A.to.B..Optical.paths.from.
A,.through.the.hole,.and.ending.at.B.difer.in.length.over.the.range.±
yr
/
L
,.where.
y
.is.the.perpendicular.
distance.of.the.hole.center.from.the.line.connecting.A.to.B.and.
r
.is.the.radius.of.the.hole..If.the.hole.
size.is.chosen.such.that.the.range.of.possible.path.lengths.of.paths.through.it.is.less.than.one-half.the.
wavelength,.then.all.the.optical.paths.through.the.hole.contribute.positively.at.B,.and.light.is.detected..
It.is.as.if.the.light.travels.from.A.to.the.hole,.changes.direction,.and.then.travels.to.B!
If.the.hole.size.radius.is.on.the.order.of.a.quarter.wavelength,.there.is.no.path.through.it.that.is.not.
within.a.half.wave.of.optical.path.length..hus,.any.quarter.wavelength.sized.hole,.on.axis.or.of.axis,.
and.located.as.far.as.of.axis.as.we.like,.will.difract.light.from.source.A.into.a.detector.at.B.
By.making.the.hole.small,.we.choked.of.all.the.paths.that.would.have.contributed.to.cancelations.at.
B.and.allowed.only.a.small.bundle.of.paths.that.have.nearly.equal.(within.one-half.wavelength).path.
lengths.to.B..his.argument.applies.no.matter.where.we.put.the.detector..So.light.emerging.from.a.small.
hole.acts.like.it.came.from.a.point.source.at.the.hole,.spreading.out.in.all.directions.
he.presence.of.a.inite.size.Fresnel.zone.is.strong.evidence.of.a.wave.explanation.to.light.propagation..
Another.is.the.presence.of.fringes.when.there.are.two.emitters..Young's.two-slit.experiment.is.the.most.
famous.example..Figure.1.7.shows.the.concept.of.the.experiment.and.the.observed.results.
Illuminating source
Slit 1
Slit 2
Detector plane
Left slit open
Observed
Both slits open
Wave theory
Particle theory
Observed
FIGuRE 1.7
Young's.two-slit.experiment..
Top
:.Young's.two.slit.experimental.set-up..
Middle
:.Observed.pattern.if.
only.the.let.slit.is.open..
Bottom
:.Both.the.right.and.let.slits.are.open..he.patterns.predicted.by.wave.and.particle.
theories.of.light..he.observed.pattern.conirms.the.wave.theory.of.light.
