1. Diffraction
Spreading of all kinds of waves from apertures and around obstacles
Illustration for VHF TV waves (60 MHz, l = 5m ) antenna placement instructions

2. Huygen’s principle
A method for describing cases where the true math is too hard. Each wave-front acts like a source of forward spherical waves.

3. Huygen’s principle and Snell’s law
Animation
Left edge of wavefront strikes first. .
Smaller circles form in the slower material.

4. Another view of Snell’s law
Peaks and valleys (phase) must match at interface. Because l is smaller in glass than air, the angles must be different.

5. Diffraction and Huygen’s principle
A wave diffracting through a “slit”. Only the slit area has Huygen’s sources for propagation of the wave. Wave spreads out through opening. Spreading angle is about D

6. Diffraction through a door

7. Diffraction is biggest when D approaches a few l’s in size

8. Diffraction through a slit
Pattern on a screen pretty far away

9. P1. Which color diffracts (spreads) the most? red blue the same
The light spreads out with places of destructive interference (dark bands)
P1. Which color diffracts (spreads) the most?
red
blue
the same

10. Diffraction of sunlight through a slit
Which color diffracts (spreads) the most?

11. Why there are dark bands
First dark band
Light from the bottom of the slit has to go farther (by Dsin q) to reach the spot on the screen at q. It’s shifted.
When the shift of light Dl from one edge is l different from the other, the average shift is l/2 (destructive), so that angle is dark.

12. P2. If I make the slit narrower, does the pattern on the screen A
P2. If I make the slit narrower, does the pattern on the screen A. contract B. expand C. stay the same

13. P3. The square diffraction pattern is shown on the left
P3. The square diffraction pattern is shown on the left. Which pattern on the right belongs to the rectangular aperture? A B

14. Diffraction through round holes

15. Diffraction through round holes
This spreading after the hole puts a limit on the resolution microscopes and telescopes, and on the resolution of our eyes.
Every star makes the same broadened shape on the screen

16. Diffraction through round holes
The bigger the diameter D, the less diffraction, and the higher the resolution.

17. Any “beam” we form must spread
Because we can only form finite width waves (beams), they must spread out due to diffraction, even if we design the rays to be parallel.

18. Multiple source interference

19. Two-slit diffraction
Each slit’s width is D. Slits are spaced by a distance d. We have a new pattern of fringes superimposed on the broader single-slit diffraction pattern
Light from the bottom slit has to go farther (by dsin q) to reach the spot on the screen at q. It’s shifted in phase.

20. Both are patterns from two slits
Both are patterns from two slits. The one with the broadest (D) slits is ____ a) top b) bottom The one with the biggest spacing (d) between slits

21. Two-slit diffraction detail
Each slit’s width is D. Slits are spaced by a distance d.

22. Diffraction grating
Instead of two slits, we have thousands of slits (or reflectors) illuminated. This gives very narrow bright spots, so we can do spectroscopy. Same equation for bright spots as for 2 slits

23. Diffraction grating and white light

24. Diffraction from
my TV screen

26. Diffraction (“structural”) colors in nature: iridescence

30. What’s missing in the albino peacock?

31. Man-made photonic Crystals: 3-D diffraction grating

32. Opals: nature’s 3-D diffraction gratings

33. Nocturnal animals
have layer of cells behind retina with thin film layers that reflect unabsorbed light back through it!

34.
Cameleon

35. Change colors for camouflage, mood
Outer layer: cells that can concentrate red or yellow pigment in center (cells almost clear), or spread them over cell (colored)
Middle layer: sacks of photonic crystal plates, reflecting blue
Inner layer: cells that can concentrate melanin (black) in center (cells almost clear), or spread them over cell (black)
Only 1 and 3 controlled by nerves?

36. Chromatophore organ

37. Iridophores (photonic crystals) and chromatophores (pigment organs)