I got this question in a university entrance exam; I'm not sure what could've been the answer. I've scoured the web and could hardly find a decent answer. The question and choices were:
What property of light allows it to propogate in space?
I simply chose "The Source" since that is how most waves are produced. Do correct me if I'm wrong. An explanation with the answer is appreciated.
Let me start by saying that as far as I know this is an odd, non-standard question. I will interpret the word “space” as outer space here; I will answer it in the context of classical E&M as that is what is tagged.
Also, I might get downvoted for this but the correct answer (out of the choices given) is the medium.
The way I am reading the question, its basically asking “why can light travel in space but other waves cant”, what’s so special about it?”. The answer to this is that light can travel through space by using the electromagnetic field as a medium, no other type of wave can do this. Relevant.
Also, the 3 other choices are basically nonesense and this question is terrible exam question, specially for a multiple choice without nuance.
Edit: I like the reasoning people are using to justify wavefront (don’t disagree) so I will say the only two nonsense choices are source and waveguide.
None of available answers suited me. There's no single property which would let alone electromagnetic waves travel in space. But I would need to name this property of EM field somehow, - it would be self-consistency or self-generation. There's a pair of Maxwell equations which best illustrates this point : $${ {\begin{array}\\\nabla \times \mathbf {E} &=-\partial _{t}\mathbf {B} \\\nabla \times \mathbf {B} &=\mu _{0}\left(\mathbf {J} +\varepsilon _{0}\partial _{t}\mathbf {E} \right)\end{array}}} $$
It reads like this. Varied electric field creates magnetic field and varied magnetic field creates electric field. So it happens that EM field is just "locked into movement" and like dancing of binary stars does not need any medium apart from space itself, - so does propagation of EM wave.
Hence, if I would be forced to choose an answer from the given set, - I would opt for "wavefront", because to some degree it summarizes this pair of Maxwell equations.
I see light as propagated in Space as being sustained in its wave form by the speed of which the photon can be pushed out of and dropped into the wave, sustaining the wave and carrying the particle. Source then might be self sustained but not origen, wave front and wave guide resultant, not causative and medium likely to be an restrictive value.
