Explain what happens when two light waves traveling from the same direction of displacement meet.

When two waves come in contact with each other while traveling on the same medium, its called constructive interference. Constructive interference happens when two waves come together.  

Ch4 and ph3 are both covalent since they are composed both of non-metals.    ch4 is non- polar because of  london forces and has a geometry of tetrahedral  ph3 , on the other hand, is umbrella shaped polar and has dipole-dipole intermolecular force (imf) . since ph3 is polar, it has a stronger imf