1. Chapter 16: Interference and Diffraction

2. Objectives Understand “Huygen’s Principle.”
Understand how waves diffract.
Explain how wavelength affects diffraction.

3. Huygen’s Principle
Christian Huygens: wavelets produce a wavefront.

4. Huygen’s Principle and Diffraction
diffraction: the bending of a wave as it passes around an obstruction or through an opening.
Huygen’s principle
helps to show how
waves diffract.
Huygen’s Principle also
shows reflection/refraction

5. Diffraction and Ocean Waves
straight wavefront
wavefront diffracts around
the edge of a jetty

7. Diffraction and Wavelength
shadows
shorter waves diffract less, forming shadows
longer waves diffract more,
virtually no shadow

8. Diffraction and Wavelength
Short light waves make shadows, but longer sound waves do not.

9. Electron Microscopes light diffracts around very small objects
electron microscopes make smaller waves that reflect

10. Objectives
Understand how interference fringes are formed by a double-slit or a diffraction grating.
Be able to calculate the wavelength of light based on an interference pattern.

11. Interference Fringes Thomas Young “proved” that light is
made of waves with his famous
“double slit” experiment.

12. “Double Slit” Diffraction Math
first order
bright
Interference can be used to calculate l :
q = tan-1 (y / L)
sinq = (l / d)
l = d·sinq
90-q n
screen
n+1 d
central
bright q l d l q

13. Diffraction Grating Each successive slit in a diffraction grating
diffracts light to form
a fringe.
n+1
n+2
n+3
l = d·sinq can be
used to calculate
wavelength

14. Calculating Wavelength
What is the wavelength of a laser if fringes
separated by 35.9 cm are made on a screen
98.2 cm from a diffraction grating with lines
spaced cm apart?

15. Objectives
Understand the importance of Thomas Young’s double-slit experiment.
Understand how interference patterns can be used to determine crystalline and molecular structures.
Understand how single-slit diffraction patterns form.

16. Thomas Young, Superstar
He read by age 2 and by age 4 had read the Bible twice; he also played several instruments.
He spoke eight languages by age 14.
As a physicist, he helped define the concept of energy, he studied the elastic properties of materials, and he explained how waves constructively and destructively interfere.
He was a practicing physician.
Based on his studies of the eye, he determined how the eye focuses and he helped develop the idea of color addition.
He was the first person to successfully use the Rosetta Stone to decipher Egyptian hieroglyphics!

17. Young’s Interference Experiment
Thomas Young used a double-slit to “prove” that light is made of waves in 1802.
Simon Poisson: monochromatic light should make a bright spot in the center of a shadow. ?

18. Diffraction and Crystal Structure
Crystal lattice structures are determined by observing patterns formed by diffracting X-rays.
Rosalind Franklin’s photo shows the X-ray diffraction pattern made by DNA. Watson and Crick stole the photo and determined the double-helix structure.

19. Single Slit Diffraction
With a single slit, dark bands are observed where destructive interference occurs.
½ l / ½ d = ½ w / L
/ d = w / 2L
w = 2L l /d
dark n
½ w
n+½
½ d L
½ l
dark

20. Resolving Power
resolving power: the ability to see two images that are close together
Airy disk

21. Objectives
Understand how iridescence occurs due to thin-films and other microscopic structures.
Understand how lasers work.

22. Iridescence
iridescence: the interference of colors caused by reflection and refraction in thin films

23. Iridescence
incident rays
in-phase
reflecting rays
out-of-phase
Rays of a single color reflect off two thin layers and cancel.
Occurs if the thickness is an odd multiple of ¼ l (¼ , ¾, etc.).
If red cancels, it appears cyan because W – R = C.
Different thicknesses result in different colors.

24. Iridescence CDs and DVDs have pitted layers that produce iridescence.
blue cancels,
yellow observed
¼ l
CDs and DVDs have pitted layers that produce iridescence.
red cancels,
cyan observed
¼ l

25. Laser Light
laser: light amplification by the stimulated emission of radiation
Typical Light
incoherent (out-of-phase)
polychromatic (many l)
Laser Light
coherent (all waves in-phase)
monochromatic (single l)
very intense
narrow beam
somewhat polarized

26. Stimulated Emission
An excited atom usually emits a photon spontaneously.
Einstein suggested that a passing photon of proper energy can stimulate an excited atom to emit a photon. The two photons will be coherent.

27. Population Inversion
Einstein said that if a majority of atoms were excited (a population inversion), a group of in-phase photons would be produced through stimulated emissions
He imagined the first laser.

28. How a Helium-Neon Laser Works
High voltage excites He atoms.
He atoms collide with Ne atoms, transfer energy, and produce a Ne population inversion.
Ne atoms undergo stimulated emissions and produce laser light.
The gases are in a long glass tube with slightly concave, mirrored ends. The laser light reflects back and forth, increasing in intensity.
One end of the tube allows 1% of the light to escape—the laser beam.

29. First Ruby Laser: 1960