Diffraction

1
Diffraction

• Waves bend when they encounter an object
• Stand in the water and watch the wake from a boat
  hit you
• Look behind you and you will see that after a
  short distance, the wake continues on
• It filled in the hole by bending around you
• This bending is called diffraction

2
Diffraction

• Diffraction is the third way to bend light
• The other two are reflection and refraction
• The amount of bending that occurs depends on the
  relative sizes of the object and the wavelength
  of the wave
• Longer wavelengths bend easier than short ones

3
Diffraction
4
Interference

• When the waves are hitting the edges of
  something, the new bending waves tend to
  interfere with each other and we get some new
  patterns
• Recall the principle of superposition
• We simply add the amplitudes

5
Interference
6
Interference

• Wave nature of light was demonstrated by Young

7
Interference

• The bright and dark areas result from differences
  in path lengths from the slits to the screen
• This changes the where the peaks and troughs
  appear
• Remember the principle of superposition

8
Interference
9
Interference
10
Interference

• Can get interference from a single slit
• Waves coming through one side of the slit
  interfere with waves coming through the other
  side
• Extend the idea to three, four slits
• Make something with hundreds of slits
• Called a diffraction grating

11
Diffraction Gratings

• Used in spectrometers (devices to separate light
  into colors
• It spreads the spectrum
• Since different colors have different
  wavelengths, the constructive interference occurs
  at different locations

12
Diffraction Gratings
13
Thin Films

• Look at an oil slick floating on the surface of a
  pond
• The slick has a rainbow of colors depending on
  the angle of viewing
• This results from interference in the very thin
  film of oil
• Called iridescence

14
Thin Films
15
Polarization

• A phenomenon that occurs in transverse waves only

16
Polarization

• These waves are plane-polarized
• All the motion is confined to a plane

17
Polarization

• Shake an electron up and down and you create an
  electromagnetic wave that is plane-polarized in
  the vertical direction
• Shake an electron side-to-side and you create an
  EM wave that is plane-polarized in the horizontal
  direction

18
Polarization

• A standard incandescent bulb emits light that is
  unpolarized
• The electrons are shaking in random directions
• So, the light has its electric field shaking in
  different directions for different waves

19
Polarization
20
Polarization

• Some transparent crystalline materials have a
  remarkable property
• These materials have their atoms arranged in
  non-cubic structures
• These crystals effectively divide the light into
  two beams that are plane-polarized at right
  angles to each other

21
Polarization
Any polarization direction can be split into a
horizontal and vertical component. Like adding
two separate beams together.
22
Polarization

• Some crystals strongly absorb one of these beams
  while letting the other beam pass right through
• These materials are called polarizers
• Take a thin sheet of such a material and imbed it
  between two sheets of glass or cellulose, and you
  have a Polaroid filter

23
Polarization
24
Polarization

• What happens if we work with two polarizers?
• If we place them so they are aligned, then the
  light passing through the first polarizer, will
  also pass through the second one
• But if the polarizers are at right angles, the
  second one will absorb all the light

25
Polarization
26
Polarization

• Most of the light reflected from non-metallic
  surfaces becomes polarized
• Consider the glare from glass or water
• Reflected wave has more vibrations parallel to
  the surface
• Analogous to skipping a rock across a pond

27
Polarization

• Sunglasses are polarized to block rays reflecting
  from horizontal surfaces light the road or a lake
• This means the direction of polarization of the
  sunglasses is vertical so it blocks horizontally
  polarized rays

28
Polarization