PHysICs :-)?

[Rosie]

Constructive interference will maximize the amplitude of two waves of identical wavelength if they are offset by:
wavelength/4
wavelength/2
wavelenth/pi
wavelength/2pi
wavelength

[Arathorn]

The whole wavelength. Then everything will be exactly doubled.

If they are offset by half the wavelength, for sinusoidal waves they will be completely cancelled out. (Results vary for other waveforms.)

[Franny]

wavelength