# Query regarding interference

In Young's Double Slit experiment, why is it that two waves vibrating perpendicular to each other doesn't show interference?

I know that for interference to happen, the waves must be coherent ( i.e., they maintain a constant phase w.r.t each other) and if the waves meet at same point in the screen it results in constructive interference and destructive if there is a phase change of lambda/2 and so on.

Any help regarding the query would be nice.

• Are you talking about polarization? If so, that's simply an observed phenomenon. – Carl Witthoft Mar 3 '14 at 13:15
• I'm talking about interference from two slits. – Albert Einstein Mar 3 '14 at 13:40

There's no way that you can add a vector pointing along the $x$ axis, and have it cancel a vector pointing in the $y$ direction. So the amplitude of the light is zero nowhere, and there are no intensity fringes. Instead, the polarization of the wave changes at the screen. As you study the light along the direction that you expect to see fringes, you would see the light changing polarization rather than amplitude: from linear (inclined 45${}^\circ$) where you expect maxima, to circular where you would expect minima. I suppose you could observe intensity fringes if you put a polarizer in from of the screen. I've never seen that done. So there is interference of a sort, it's just of a different kind.