Question

What is the minimum distance between two point in a wave having phase difference 2pi

Answer 1

It will have to be one wavelength.

Phase difference Δϕ between two points on a wave is given by

Δϕ=Δxλ×2π

where λ is the wavelength

x: distance between the two points

Now if you substitute Δϕ=2π you will end up with Δx=λ.

Answer 2

It is one wave length