Light’s Nature• Wave nature (electromagnetic wave)• Particle nature (bundles of energy calledphotons)
Past- Separate Theories of EitherWave or Particle Nature• Corpuscular theory of Newton (1670)• Light corpuscles have mass and travel atextremely high speeds in straight lines• Huygens (1680)• Wavelets-each point on a wavefront actsas a source for the next wavefront
Why was it difficult to prove thewave part of the nature of light?
Proofs of Wave Nature• Thomas Youngs Double Slit Experiment (1807)bright (constructive) and dark (destructive)fringes seen on screen• Thin Film Interference Patterns• Poisson/Arago Spot (1820)• Diffraction fringes seen within and around asmall obstacle or through a narrow opening
Proof of Particle Nature:The Photoelectric Effect• Albert Einstein 1905• Light energy is quantized• Photon is a quantum or packet of energy
The Photoelectric Effect• Heinrich Hertz first observed thephotoelectric effect in 1887• Einstein explained it in 1905 and won theNobel prize for this.
Thomas Young’s Double SlitInterference Experiment• Showed an interferencepattern• Measured thewavelength of the light
For ConstructiveInterference:The waves must arriveto the point of study inphase.So their path differencemust be integralmultiples of thewavelength:∆L= nλn=0,1,2,3,………
For destructive interference:, the waves mustarrive to the point ofstudy out of phase.So the path differencemust be an oddmultiple of λ/2:∆L= nλ m=1/2,3/2,
Typical Question• Where is the first location of constructiveor destructive interference?
Fo Constructive Interference of Waves fromTwo Sourcesx=Ltanθsinθ= ∆L/d∆L=nλFor small angles:Lsinθ~Ltanθdsinθ=nλnλ = dxLdθLxθn=0,1,2,3,…
Double Slit Interferencedsinθ=nλnλ = dxLConstructive (brights) n=0,1,2,3,…..Destructive (darks) n=1/2, 3/2, 5/2,…..Note:To find maximum # of fringes set θ to 90ofor n.
Question• How does x change with wavelength?• How does x change with slit distance?
ProblemTwo slits are 0.05 m apart. A laser ofwavelength 633nm is incident to the slits.A screen is placed 2m from the slits.a) Calculate the position of the first andsecond bright fringe.b) What is the maximum number ofdestructive interference spots there can beon either side of the central maximum?
Diffraction Grating• Large number of equally spaced parallel slits.• Equations are same as for double slit interferencebut first calculate the d (slit separation) from thegrating density, N.d=1/N , N slits per unit lengthdsinθ=nλnλ = dxLConstructive (brights) n=0,1,2,3,…..Destructive (darks) n=1/2, 3/2, 5/2,…..
ProblemA neon laser of wavelength 633nm is pointedat a diffraction grating of 3000lines/cm. Findthe angle where the first bright occurs.(Hint: slit separation d is inverse of gratingdensity)
DiffractionWave bends as it passes an obstacle.
Diffraction through a Narrow SlitEach part of the slit acts as a point sourcethat interferes with the others.(Based on Huygens Principle)
Diffraction from Narrow Slitwsinθ=nλλ= nw yLw: is the width of the slitDestructive (dark fringes): m=0,1,2,3,….
Questions• How does x change with the width?• How does x change with the wavelength