Interference and Diffraction

1
Chapter 16

• Interference and Diffraction

2
Chapter 16 section 1

• Objectives
• Describe how light waves interfere with each
  other to produce bright and dark fringes.
• Identify the conditions required for interference
  to occur.
• Predict the location of interference fringes
  using the equation for double-slit interference.

3
Interference

• waves can superpose one another, bend around
  corners, reflect off surfaces, be absorbed, and
  change direction when entering new material
• All of these characteristics vary with the
  wavelength of light
• interference takes place only btwn waves w/the
  same wavelength
• End up w/a resultant wave

4
Interference

• light waves w/the same wavelength are
  monochromatic
• Single colored
• Constructive interference equals an increase in
  brightness
• Destructive interference equals dimmer spots
• for interference to be seen, there must be a
  constant phase difference and light must be
  monochromatic

5
In phase

• Crest of one wave overlaps the crest of another
  wave
• Phase difference is 0 degrees

6
Out of Phase

• crest of one wave overlaps the trough of another
  wave
• phase difference is equal to 180 degrees

7
Coherent and Incoherent

• if phase difference stays constant the waves are
  said to be coherent
• if phase difference is not constant btwn waves,
  no interference pattern is observed and waves are
  incoherent

8
Demonstration of Interference

• interference can be demonstrated by using
  parallel slits
• Result is bright and dark parallel bands
• Bright constructive interference
• Dark destructive interference
• at the center of the screen, waves travel the
  same distance and arrive in phase (bright)
• off the center, some waves are more distant and
  must travel ½ a wavelength farther and are out of
  phase (dark)

9
White Light

• is more difficult b/c it has waves of many
  different wavelengths
• Pattern is less distinct b/o the many colors
• Tends to be blurred and indistinct
• ex. colors on soap bubbles

10
Path of Light

• when using parallel slits, the distance btwn the
  two slits determines whether waves are in phase
  or not
• the distance btwn the slits can be used to
  measure the wavelength of light
• Each wave must travel a distance from the slit to
  the screen
• Difference is equal to d(sin ?)
• d is the distance btwn slits

11
Constructive Interference

• path difference determines where the two waves
  are in or out of phase at the screen
• If path difference is 0 or a whole number, they
  are in phase (constructive)
• d(sin ?) m? m 0, 1,2 ..
• m order
• 0 the central maximum
• 1 the 1st order maximum, etc.
• Dark areas indicate minima

12
Destructive Interference

• if the path difference is an odd number, the
  waves are out of phase (destructive)
• d(sin ?) (m ½)? m 0, 1,2

13
Review and Assignment

• Describe how light waves interfere with each
  other to produce bright and dark fringes.
• Identify the conditions required for interference
  to occur.
• Predict the location of interference fringes
  using the equation for double-slit interference.
• Page 603 1 4

14
Chapter 16 section 2

• Objectives
• Describe how light waves diffract around
  obstacles and produce bright and dark fringes.
• Calculate the positions of fringes for a
  diffraction grating.

15
Diffraction

• spreading of waves into a region behind an
  obstruction
• Waves bend around obstacles
• ex. hearing someone around the corner
• Usually occurs when waves pass through small
  openings, around obstacles, or sharp edges
• When the slit is narrowed, light begins to spread
  out and produce a diffraction pattern

16
Diffraction

• results from constructive and destructive
  interference
• each point on the slit acts as a source of waves
• intensity d/o the angle of diffraction
• Diffraction from monochromatic light through a
  single slit consists of a broad central band
  (central max.), flanked by a series of narrower
  less intense bands

17
Diffraction Gratings

• diffraction grating can transmit/reflect light
• Uses diffraction or interference to disperse
  light into separate color
• Consists of many parallel slits
• d(sin ?) m? m 0, 1, 2
• d space btwn slits
• All wavelengths combine at 0 or m 0 (zero order)

18
Diffraction Gratings

• sharpness and range d/o number of lines in
  grating
• number of lines in a grating is the inverse of
  the line separation (1/d)
• 5000 lines/cm d 1/5000
• if the number of lines on a grating increases,
  the separation decreases and the farther apart
  the wavelengths are
• diffraction gratings are used in spectrometers
  to study chemical composition and temperature of
  stars

19
Review and Assignment

• Describe how light waves diffract around
  obstacles and produce bright and dark fringes.
• Calculate the positions of fringes for a
  diffraction grating.
• Page 610 1 5

20
Chapter 16 section 2/3

• Objectives
• Describe how diffraction determines an optical
  instruments ability to resolve images.
• Describe the properties of laser light.
• Explain how laser light has particular advantages
  in certain applications.

21
Instruments that use Diffraction

• microscopes and telescopes use diffraction
• Each object seen has a bright center flanked by
  weaker bright and dark rings
• If objects are close to each other, the
  diffraction patterns overlap
• if objects are separated enough that the central
  maxima do not overlap, their images can barely be
  resolved
• to get high resolution, the angle btwn the
  objects should be as small as possible

22
Resolving Power

• ability of an optical instrument to separate two
  images that are close together
• shorter the wavelength of light or the wider the
  opening of the aperature, the smaller the angle
  of resolution
• if you increase resolving power it means a
  smaller portion of the object will be seen
• ex. magnifying glass

23
Rayleigh Criterion

• ? 1.22(?/D)
• D is equal to the diameter of the lens
• a small aperature is fine for shorter ?
  (x-rays)
• a larger aperature is needed for longer ?
  (radio)
• resolution power can be limited by air particles
  in the atmosphere

24
Lasers

• Light Amplication by Stimulated Emission of
  Radiation
• device that produces an intense, nearly parallel
  beam of coherent light
• waves are in phase and do not shift relative to
  each other
• b/c waves are in phase, they interfere
  constructively
• waves behave like a single wave w/a very large
  amplitude
• light is monochromatic
• b/o these properties, intensity is greater
• E/time/area

25
Lasers

• very high E light source
• lasers convert light, electrical E, or chemical
  E into coherent light
• use an active medium and add E to it to produce
  coherent light
• Active medium can be solid, liquid, or gas
• Composition of medium determines wavelength or
  color of light produced

26
How lasers work

• 1. when E is added to active medium, some E is
  absorbed by atoms and then released in the form
  of light waves
• 2. atoms then return to original E level
• 3. these initial waves cause other atoms to
  release their E w/same wavelength, phase, and
  direction (stimulated emission)
• 4. mirrors reflect waves back into medium and
  increase intensity
• Laser beams are very narrow

27
Uses of lasers

• 1. measure distance by time it takes to travel
  to object
• 2. in DVD and CD players
• 3. medical procedures
• Remove scars (body absorbs wavelength)
• Cutting (vaporize water and heat)
• Coagulates blood
• Eye surgery
• 4. read bar codes

28
Review and Assignment

• Describe how light waves diffract around
  obstacles and produce bright and dark fringes.
• Calculate the positions of fringes for a
  diffraction grating.
• Page 612 1 6 and 618 1 - 3