2. Light’s Nature
• Wave nature (electromagnetic wave)
• Particle nature (bundles of energy called
photons)
3. Past- Separate Theories of Either
Wave or Particle Nature
• Corpuscular theory of Newton (1670)
• Light corpuscles have mass and travel at
extremely high speeds in straight lines
• Huygens (1680)
• Wavelets-each point on a wavefront acts
as a source for the next wavefront
4. Why was it difficult to prove the
wave part of the nature of light?
5. Proofs of Wave Nature
• Thomas Young's 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 a
small obstacle or through a narrow opening
6. Proof of Particle Nature:
The Photoelectric Effect
• Albert Einstein 1905
• Light energy is quantized
• Photon is a quantum or packet of energy
7. The Photoelectric Effect
• Heinrich Hertz first observed the
photoelectric effect in 1887
• Einstein explained it in 1905 and won the
Nobel prize for this.
8. Thomas Young’s Double Slit
Interference Experiment
• Showed an interference
pattern
• Measured the
wavelength of the light
12. For Constructive
Interference:
The waves must arrive
to the point of study in
phase.
So their path difference
must be integral
multiples of the
wavelength:
∆L= nλ
n=0,1,2,3,………
13. For destructive interference:
, the waves must
arrive to the point of
study out of phase.
So the path difference
must be an odd
multiple of λ/2:
∆L= n
λ m=1/2,3/2,
15. Fo Constructive Interference of Waves from
Two Sources
x=Ltanθ
sinθ= ∆L/d
∆L=nλ
For small angles:
Lsinθ~Ltanθ
dsinθ=nλ
nλ = dx
L
d
θ
L
x
θ
n=0,1,2,3,…
16. Double Slit Interference
dsinθ=nλ
nλ = dx
L
Constructive (brights) n=0,1,2,3,…..
Destructive (darks) n=1/2, 3/2, 5/2,…..
Note:
To find maximum # of fringes set θ to 90o
for n.
17. Question
• How does x change with wavelength?
• How does x change with slit distance?
18. Problem
Two slits are 0.05 m apart. A laser of
wavelength 633nm is incident to the slits.
A screen is placed 2m from the slits.
a) Calculate the position of the first and
second bright fringe.
b) What is the maximum number of
destructive interference spots there can be
on either side of the central maximum?
20. Diffraction Grating
• Large number of equally spaced parallel slits.
• Equations are same as for double slit interference
but first calculate the d (slit separation) from the
grating density, N.
d=1/N , N slits per unit length
dsinθ=nλ
nλ = dx
L
Constructive (brights) n=0,1,2,3,…..
Destructive (darks) n=1/2, 3/2, 5/2,…..
21. Problem
A neon laser of wavelength 633nm is pointed
at a diffraction grating of 3000lines/cm. Find
the angle where the first bright occurs.
(Hint: slit separation d is inverse of grating
density)
