Two-source interference: the four demonstrations
Water: two dippers on one bar in a ripple tank give anti-nodal lines of large amplitude and nodal lines of calm water. Sound: two loudspeakers on one signal generator, walk along a line in front and hear alternating loud and soft. Light: Young's double slit, giving bright and dark fringes on a screen. Microwaves: a transmitter feeding two slits cut in an aluminium plate, with a detector and meter reading alternating maxima and minima as it moves along a line.
What this actually means
One shared setup runs through all four: a single source split into two, then a detector swept along a line in the overlap region. Describe it that way and the marks come easily.
In every case say explicitly how coherence is achieved: one vibrating bar for the two dippers, one signal generator for the two speakers, one single slit or laser for the two slits, one transmitter for the two gaps.
Match the observable to the wave. Water gives large-amplitude and calm regions; sound gives loud and soft; light gives bright and dark fringes; microwaves give maximum and minimum detector readings.
The wording trap is real. For microwaves you must not write bright and dark or loud and soft. For sound you must not write bright and dark either. Markers do take those off.
Sound and microwaves have conveniently large wavelengths (about a metre and a few centimetres) so maxima can be metres apart and easy to walk between. Light needs a screen a metre away and slits half a millimetre apart to get fringes you can see at all.
Describing microwave or sound interference using the words bright and dark fringes.
Prove it — watch it be true
- Set the source wavelength to the microwave range and note how far apart the maxima are on the screen profile.
- Switch the wavelength to visible light with the same slit separation and confirm the fringes crowd together.
- In each case drag the path-difference probe to a maximum and check it reads a whole number of wavelengths.