Optical Interference
Optical interference is the redistribution of light intensity that results when coherent waves superpose, producing bright and dark fringes set by their relative phase.
Definition
The phenomenon in which the intensity of superposed coherent light waves varies with position according to their phase difference, yielding maxima where the waves reinforce and minima where they cancel.
Scope
This topic covers the superposition of two or more coherent light waves and the resulting fringe patterns. It includes Young's double-slit experiment, division-of-wavefront and division-of-amplitude arrangements, the dependence of fringe position and spacing on path difference and wavelength, multiple-beam interference and its sharp fringes, and the relation between fringe visibility and coherence. It treats the conditions for constructive and destructive interference and the conservation of energy through redistribution rather than loss.
Core questions
- What conditions on phase produce bright and dark interference fringes?
- How does fringe spacing depend on geometry and wavelength?
- How is interference produced by dividing a wavefront versus dividing an amplitude?
- How does multiple-beam interference sharpen the fringes?
Key concepts
- phase difference
- optical path difference
- constructive and destructive interference
- fringe spacing
- fringe visibility
- division of wavefront
- division of amplitude
- Fabry-Perot interference
Key theories
- Two-beam interference
- Two coherent waves combine to give an intensity that depends on the cosine of their phase difference, producing evenly spaced fringes; Young's double slit is the archetypal example linking fringe spacing to wavelength and geometry.
- Multiple-beam interference
- When many beams of progressively decreasing amplitude interfere, as between two highly reflective surfaces, the transmission shows very narrow, sharp peaks exploited in the Fabry-Perot interferometer.
Clinical relevance
Interference is the working principle of optical coherence tomography, which produces cross-sectional images of the retina and other tissues by measuring the interference of light reflected from different depths.
History
Young demonstrated interference of light with his double-slit experiment around 1801, providing strong evidence for the wave theory against Newton's corpuscular view. Fresnel extended the analysis, and at the close of the nineteenth century Fabry and Perot devised the multiple-beam interferometer that bears their names, enabling high-resolution spectroscopy.
Key figures
- Thomas Young
- Augustin-Jean Fresnel
- Charles Fabry
- Alfred Perot
Related topics
Seminal works
- hecht2017
- bornwolf1999
Frequently asked questions
- Does interference destroy light energy where the fringes are dark?
- No; energy is conserved and merely redistributed, so the light removed from the dark fringes reappears as extra brightness in the bright fringes.
- Why must the sources be coherent for stable fringes?
- Stable fringes require a constant phase relationship between the interfering waves; if the relative phase fluctuates rapidly, as with independent sources, the fringe pattern shifts too quickly to be observed and averages out.