2

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!

Hiiii
  • 1,113
  • 1
    Possible duplicate ??http://physics.stackexchange.com/questions/3164/how-is-a-spherical-electromagnetic-wave-emitted-from-an-antenna-described-in-ter?rq=1 –  Feb 23 '17 at 17:43
  • That link seems to explain it nicely. Thank you (: – Hiiii Feb 23 '17 at 17:48

2 Answers2

3

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.

anna v
  • 233,453
  • 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)... – Hiiii Feb 23 '17 at 18:18
2

It would help if you imagine a lot of photons propagating in all directions. Each photon propagating in a straight line.

Ivan
  • 960
  • 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... – Hiiii Feb 23 '17 at 18:20