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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!

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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.

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  • $\begingroup$ Ahh they are making the dispersion small by using special materials. Thank you, that link is helpful (: I'm still going through it. Is it correct to think that the source of any light is an atom( change in elctron state) ? I guess we can assume the atoms are like point sources (1 Angstrom)... $\endgroup$ – Hiiii Feb 23 '17 at 18:18
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It would help if you imagine a lot of photons propagating in all directions. Each photon propagating in a straight line.

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  • $\begingroup$ Thank you. I'm wondering what happens if my laser shoots only one photon. Would it propagate as an expanding sphere or take a straight path... $\endgroup$ – Hiiii Feb 23 '17 at 18:20

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