Question

like in a sound wave crest represents compression and trough represents rarefaction then what do crest and trough represents in an Electromagnetic wave

Answer 1

actually
a changing electric field produces a changing magnetic field and vice-versa which gives rise to transverse wave known as electromagnetic wave. it means electromagnetic are tranverse wave.but you know sound is longitudinal wave. their particle moves along wave motion.so, in case of electromagnetic wave we can't say crest is compression , and trough is rarefraction.