D Incident wave 2 P b T (a) B C Ꮎ Te 71 (b) St 196 Path length difference AL (a) Waves from slits S₁ and S2 (which extend into and out of the page) combine at P, an arbitrary point on screen C at distance y from the central axis. The angle serves as a convenient locator for P. (b) For Dd, we can approximate rays r₁ and r₂ as being parallel, at angle to the central axis.