1. Interference Diffraction and Lasers
Chapter 15

2. Interference of Light
Superposition of 2 identical wavetrains traveling in same or opposite directions
Property of all waves, longitudinal and transverse, including light
First shown by Thomas Young in 1801 by passing monochromatic light through two narrow slits
Results in areas of increased and decreased intensity

3. Interference patterns
Interference with monochromatic light produces alternate light and dark bands called fringes
Bright fringes are caused by constructive interference, with waves in phase
Dark fringes are caused by destructive interference, waves out of phase

4. Double Slit Interference
Light passing through two narrow slits diffracts and overlaps producing interference pattern on screen
For constructive interference, path difference equals whole-number multiple of wavelength:
For destructive interference path difference must be odd number of half wavelengths:

6. Double Slit Interference

9. Thin Film Interference
Light reflects from top and bottom surface of thin, transparent film
Each reflection travels different distance, so interference results, depending on thickness of film
Some wavelengths are canceled, some reinforced

10. Thin Film Interference
Result is swirling rainbow effect seen in soap bubbles, gasoline on water, etc.
When distance difference is 1/2 l, (3/2, 5/2, etc.) constructive interference occurs - phase is reversed in one reflected ray
When distance difference is 1l, (2, 3, etc.) destructive interference occurs, color is canceled, comp. color seen

11. Uses of Interference
Regular surfaces produce regular interference patterns
Used to check measurements, tolerances, etc.
Interferometer uses interference patterns to make precise distance measurements

12. Huygen’s Principle
Waves spreading from point source are made of many overlapping small waves
Every point on the wave is a point source of secondary waves
Explains diffraction
Christian Huygens

13. Diffraction
Spreading of a wave into area beyond barrier or small opening
Causes wave to bend
Occurs in all waves
More pronounced when obstruction or opening is small compared to wavelength

14. Diffraction
Long e-m waves easily diffracted around buildings, hills, etc. (AM radio)
Visible light diffracted by objects around 10-7 m; determines limit of optical microscope
Electron beam has shorter wavelength so electron microscopes can resolve much smaller objects

15. Diffraction of Light
1816: Fresnel explained diffraction with interference
Diffraction through double slit or single slit both cause interference, slightly different pattern

16. Diffraction Gratings
Transmission grating: transparent film with many evenly spaced fine lines
Reflection grating: reflective surface with many evenly spaced grooves
Diffraction angle depends on wavelength so light is dispersed showing spectrum
Interference causes spectrum to be repeated

17. Diffraction Calculations
Grating constant (d) is distance between lines on the grating
n is number of spectrum
qn = Diffraction angle of each spectrum
For first order spectrum, (n = 1) l = d sinq
For any other spectrum, l = (d sinqn)/n

18. Lasers Stands for: Light Amplified by Stimulated Emission of Radiation
Emit coherent light: same direction, frequency, phase
Ordinary light sources are incoherent: chaotic, mixed frequencies, no phase relationship, all directions

19. Spontaneous Emission
Energy is absorbed by atoms causing electrons to move to higher energy levels
Atom is in excited state
Electrons fall back to normal levels emitting photons of light
Atom returns to ground state

20. Stimulated Emission Excited states are usually very unstable
Many materials can be brought to slightly stable (metastable) energized state
Controlled energy input can create a population inversion where more atoms are in metastable excited state than in ground state.

21. Stimulated Emission
Spontaneous emission of one photon causes avalanche of identical photons through chain reaction
All photons have same energy and frequency, so light is monochromatic

22. Laser Construction
Lasing cavity is shaped for resonance at desired frequency; emissions at other frequencies quickly die out
Energy input from electricity or light flashes excites lasing medium
Mirrors at each end reflect laser light back through medium amplifying beam

23. Laser Construction
Mirror at one end weakly silvered so beam can escape when strong enough
Some lasers pulse, some continuous
Many lasing materials discovered, gases, liquids, dyes, solids, semiconductors, crystals, etc.

24. Holograms Produced by interference of coherent light, gives 3-D image
Beam is split with one half going directly to film, other half reflects off subject
Since beams travel different distances, interference occurs
Interference pattern produced on film