I know light travels in straight lines so that the momentum is conserved ($p=h/\lambda$).
However in some derivations I also see that electromagnetic waves propagate as spherical waves, like an expanding balloon.
I'm a bit confused by these two contradicting explanations. I'm not that good at physics. Has this something to do with wave-particle duality ? If so, should we assume one photon as one expanding balloon ? Appreciate any help in clearing this up for me. Thanks!