Phase Points on a wave which are always travelling in the same direction, rising a falling together, are in phase with each other.
Points on a wave which are always traveling in opposite directions to each other, one is rising while the other is falling, are in antiphase with each other.
Path difference
If we measure the distance travelled by two waves and then compare those distances, any difference in the distances travelled is called the path difference. Path difference is measured in metres (m).
Transverse waves can be polarised but longitudinal waves cannot. Unpolarised light is a mixture if waves in different planes. When this light is passed through a polaroid material only light waves in one plane are transmited and the light is now polarised. Polaroid sunglasses are popular with fishermen they reduce glare by blocking the reflected polarised light from the waters surface.