Interference in the Double-Slit Experiment

• Light from a single source which passes through two slits can interfere with itself
• The result is a pattern of bright bands and dark bands on some surface far from the slit mask
• Bright bands occur where waves from the two slits reach the surface in phase with each other; dark bands occur where the waves are out of phase
• The general formula is

            d sin(theta) = m         lambda         constructive interference
  
            d sin(theta) = (m + 1/2) lambda         destructive interference
  

• The central bright spot is brightest, but the maxima near it are only slightly less bright
• Light which partially passes through and partially reflects off a thin film of material can also interfere with itself
• If a light ray in one substance reflects off an interface to a material with a higher index of refraction, its phase is reversed
• If a ray which reflects off the front surface a film, and a ray which reflects off the back surface of a film, recombine in phase, one sees the reflected light; if they recombine out of phase, no reflection appears
• The reflection from a thin film can depend strongly on wavelength

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