2 Source Interference

• Coherent wave sources


  Sources of waves that oscillate at the same frequency with a constant phase relationship (difference)
• Monochromatic sources


  Sources of waves of one frequency
• Two waves always interfere, even if the result of the addition is complicated or not remarkable but two coherent sources of waves produce a pattern of interference
• Observing interference is evidence of wave-like behaviour
• Two source interference


  1. Monochromatic light through two slits to produce two coherent sources

  2. Interference pattern of constructive and destructive interference observed on a screen
• In the lines of destructive interference there is often near complete cancellation
• With light waves this produces a series of bright and dark fringes on a screen
• With sound this would produce a series of loud and quiet regions in a room
• Points of cancellation


  Destructive interference
• Points of reinforcement


  Constructive interference
• Laser can be used as a monochromatic source without requiring a monochromatic filter and a single slit
• Low power lasers can damage your retina if you look directly into them, Always project onto a screen
• Interference patterns only occur with coherent sources, incoherent sources do not produce interference patterns
• Path difference


  Difference in distance travelled by waves from two sources
• Interference patterns support the wave nature of light