1. The two sources must be coherent.
2. The sources should emit light of same frequency.
3. The amplitudes of two wave must be equal otherwise a clear cut distinction between maximum (a1 + a2) and minimum (a1 – a2) is not possible.
4. The two waves must propagate along the same line.
5. The two sources must be narrow otherwise a single source may itself be equivalent
to a large number of fine sources which will give their own pattern.
6. The separation between sources should be small so that wide fringes are obtained.
7. Separation between source and screen should be large to get wide fringes.
8. If the interfering waves are polarised they must be in the same state of polarisation.