Conditions for observable two-source fringes
● VERBATIM — examined word-for-wordTo observe a steady two-source interference pattern: (1) the sources must be coherent, that is have a constant phase difference and therefore the same frequency; (2) the waves must be of the same type and must overlap in the region of observation; (3) the amplitudes should be approximately equal, for good contrast; (4) for electromagnetic waves, they must be unpolarised or polarised in the same plane. For light in particular the slit separation must be small enough that the fringes are wide enough to see.
What this actually means
Coherence is the condition that actually decides whether a pattern exists. The others decide whether you can see it clearly, which is why the question usually says observed fringes rather than interference.
Amplitude matching controls contrast. Equal amplitudes give completely dark minima. If one slit is partly covered, the bright fringes dim slightly and the dark fringes become grey, so contrast falls even though the spacing is unchanged.
The polarisation condition only bites for transverse waves. Two beams polarised at 90° to each other superpose but produce no intensity variation, so no fringes. Never quote this condition for sound.
Overlap sounds trivial until you meet the question where the two beams have diverged so far apart that they no longer share a region of space. No overlap, no pattern.
For light, if a is too large then x = λD/a becomes microscopically small and the fringes merge. This is why you cannot do Young's experiment with two slits a centimetre apart. Sources must also be separated by more than one wavelength.
Listing polarisation as a condition for sound waves, which are longitudinal and cannot be polarised.
Prove it — watch it be true
- Start with coherent, equal-amplitude sources and note the high-contrast fringes on the screen intensity profile.
- Use the coherence-breaking toggle to randomise phase and watch the fringes disappear.
- Restore coherence but reduce the second source amplitude, and confirm the minima lift off zero so contrast drops while spacing stays the same.