1. Light exhibits both wave and particle properties, which was difficult to explain until the photoelectric effect showed light behaves as quantized packets of energy called photons. 2. Thomas Young's double slit experiment provided early evidence of light's wave nature by producing interference patterns of bright and dark fringes that could only be explained by wave theory. 3. Other experiments like thin film interference and diffraction around obstacles further supported light's wave nature by demonstrating phenomena like interference and diffraction of waves that can be explained using wave equations and models.

1. Interference and Diffraction
Physics
Mr. Berman

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

9. Two Waves Interfering

10. Young’s Double Slit
Interference Pattern

11. http://galileo.phys.virginia.edu/classes/USEM/SciImg/home_files/introduction_files/doubleslit.jpg

12. Interference of Waves From
Two Sources
• Simulation “Ripple Tank”
http://www3.interscience.wiley.com:8100/l
egacy/college/halliday/0471320005/simula
tions6e/index.htm?newwindow=true

13. Interference
• Young’s Double Slit Interference
• http://galileo.phys.virginia.edu/classes
/109N/more_stuff/flashlets/youngexpt
4.htm

14. 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:
DL= nl
n=0,1,2,3,………

15. 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 l/2:
DL= n l
m=1/2,3/2,5/2,….

16. Typical Question
• Where is the first location of constructive
or destructive interference?

17. Fo Constructive Interference of Waves from
Two Sources
x=Ltanq
sinq= DL/d
DL=nl
For small angles:
Lsinq~Ltanq
dsinq=nl
nl = dx
L
d
q
L
x
q
n=0,1,2,3,…

18. Double Slit Interference
dsinq=nl
nl = 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 q to 90o for n.

19. Question
• How does x change with wavelength?
• How does x change with slit distance?

20. 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?

21. Diffraction Grating
http://des.memphis.edu/lurbano/vpython/matter_interactions/spectrum/spectrum_02.jpg

22. 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
dsinq=nl
nl = dx
L
Constructive (brights) n=0,1,2,3,…..
Destructive (darks) n=1/2, 3/2, 5/2,…..

23. 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)

24. Diffraction
Wave bends as it passes an obstacle.

25. Diffraction through a Narrow Slit
Each part of the slit acts as a point source
that interferes with the others.
(Based on Huygens Principle)

26. Pattern of Diffraction of Light
through a Narrow Slit
L
w
x

27. Intensity of the Diffraction
Interference Patterns
• Simulation “Interference of Light”
• http://www3.interscience.wiley.com:8100/l
egacy/college/halliday/0471320005/simula
tions6e/index.htm?newwindow=true

28. Diffraction from Narrow Slit
wsinq=nl
l= nw y
L
w: is the width of the slit
Destructive (dark fringes): m=0,1,2,3,….

29. Questions
• How does x change with the width?
• How does x change with the wavelength

30. Diffraction around a Penny and
Poison Spot

31. Example of Diffraction