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!
It is a matter of the boundary conditions. A spherical light wave comes from a point hole, or a point source. The light waves of the sun , since a very small angle is subtended, can be considered a plane wave.
The light from a laser comes from a particular construction of crystals and reflective surfaces so that the light is coherent as a plane wave and has very small dispersion.
It would help if you imagine a lot of photons propagating in all directions. Each photon propagating in a straight line.
