Intensity in interference
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Intensity distribution across the fringes.
I = 4I₀cos²(φ/2)I = 4I₀cos²(φ/2) for two equal sources, varying between 0 and 4I₀. Average intensity is 2I₀, so energy is redistributed, not created.
Bright fringes are 4 times the single-slit intensity.
Intensity ∝ amplitude squared.
In more detail
The intensity in an interference pattern varies between maximum and minimum values, depending on the phase difference. The maximum intensity is four times that of a single slit for equal slits.
Real-world uses
- Interference patterns are used to analyse surfaces and coatings.
- Engineers design optical systems using intensity distributions.
- Physicists use the intensity pattern from two coherent sources to predict the brightness at each point of a screen.
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Frequently asked questions
Why four times?
Because amplitudes add, and the intensity of a single source is proportional to amplitude squared.
Can the intensity be zero?
Yes, at points of destructive interference for equal sources.
Why do the bright fringes have the same intensity?
In an ideal double slit, the fringes repeat with equal maximum intensity, though the single-slit envelope can reduce the outer fringes.
Related topics in Optics
Coherent sources · Young's double slit · Fringe width · Thin film interference · Newton's rings · Michelson interferometer