In 30 seconds
This 3Blue1Brown excerpt uses a laser and a simplified wave model to explain double-slit interference. Equal paths reinforce at the center, while a half-wavelength path difference can produce cancellation.
- 1Equal pathsAt the center, waves arriving in phase reinforce each other.
- 2Half a wavelengthA half-wavelength path difference can produce cancellation.
- 3White lightDifferent wavelengths overlap; suitable conditions can still reveal colored fringes.
Takeaways
- Two coherent wave paths can reinforce or cancel depending on their path difference.
- Opening a second slit can make a particular point darker by destructive interference.
- Fringe positions depend on wavelength, so white light produces overlapping color patterns.
Read the edited text by chapter
The double-slit demonstration
In this excerpt from a longer video about holograms, the narrator introduces double-slit interference using a laser demonstration at the Exploratorium. Instead of just two bright spots, a screen shows a series of bright and dark regions.
In this excerpt from a longer video about holograms, the narrator introduces double-slit interference using a laser demonstration at the Exploratorium. Instead of just two bright spots, a screen shows a series of bright and dark regions.
Waves reinforce and cancel
For a simplified model, each narrow slit acts like a source of waves with a single wavelength. At the center of the screen, the distances from the two slits are equal, so the waves arrive in phase and reinforce one another.
For a simplified model, each narrow slit acts like a source of waves with a single wavelength. At the center of the screen, the distances from the two slits are equal, so the waves arrive in phase and reinforce one another.
At a point where one path is half a wavelength longer, a peak can meet a trough. With equal amplitudes the waves cancel, creating a dark fringe: opening a second slit can make that particular point darker.
Wavelength and the pattern
Moving across the screen changes the path difference, alternately bringing the waves into and out of phase. This produces the alternating bright and dark pattern; adding light redistributes intensity rather than making every point brighter.
Moving across the screen changes the path difference, alternately bringing the waves into and out of phase. This produces the alternating bright and dark pattern; adding light redistributes intensity rather than making every point brighter.
Different wavelengths produce fringes at different positions, so their patterns overlap in white light. With suitable conditions, white light can still produce a central white fringe and nearby colored fringes; the monochromatic pattern does not simply disappear in every setup.
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