Coherence and Young's Double-Slit Fringes
Two coherent sources give evenly spaced fringes whose spacing follows w = lambda D over s. A bright fringe sits wherever the path difference is a whole number of wavelengths.
What a learner can do afterwards
- Explains why the two slits must be lit from one source, and what coherent means here
- Uses the fringe spacing equation to get a wavelength from measurements on a screen
- Predicts what happens to the pattern when the slit separation, the screen distance or the colour changes
1 · Read
Coherent sources share one frequency with a fixed phase link over time. Young lit both slits from a single slit, since separate bulbs fire at random and never hold still.
Light spreads from each slit and overlaps on the screen. Where crest meets crest a bright fringe grows, and where crest meets trough they cancel dark. Bright fringes sit where the path gap is a whole number of wavelengths.
Fringe spacing follows w equals lambda D over s, with D to the screen and s the slit gap. Measure w, D and s, then solve for the wavelength of the laser.
Wider slit gaps squeeze fringes together, while longer screens or redder light spread them out. Red spreads wider than blue, since longer wavelengths spread the pattern more.
One source keeps the slits coherent, and the fringe spacing reveals the wavelength.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.