When do you actually see fringes?

Interference happens whenever two waves overlap. Seeing a stable pattern of fringes is harder. On a screen, fringes are alternating bright and dark bands, and for them to be visible several things must be true at once.

Miss any one of them and the pattern either vanishes, blurs into uniform light, or shifts too fast to see.

The two sources must have the same frequency and a constant phase difference. If the phase difference drifts, the maxima move, and the screen shows a blur rather than fringes.

In practice this means both waves come from one source — two slits lit by one lamp, or two loudspeakers on one signal generator.

If the two waves have very different amplitudes, the cancellation at a minimum is only partial. The dark fringes become grey and the pattern loses contrast.

Equal amplitudes give complete cancellation, so the minima are truly dark and the fringes stand out. This affects contrast, not the positions of the fringes.

Real light is not a perfect sine wave; it comes in short bursts, and its phase is only predictable over a certain distance called the coherence length.

If the path difference between the two waves is larger than the coherence length, the waves arrive with an unpredictable phase difference and the fringes wash out.

Using monochromatic light — one wavelength, ideally from a laser — keeps the fringes sharp. White light contains many wavelengths; each colour makes its own pattern at a slightly different spacing, so away from the centre they overlap and blur into white.

The fringes must be separated by more than the width of a detector or the eye can resolve. Fringe spacing grows when:

• the slits are closer together,
• the screen is further away,
• the wavelength is longer.

That is why light needs slits a fraction of a millimetre apart and a screen a metre or more away. Microwaves, with centimetre wavelengths, need far less.