[Eternal Student]

Hi TommyJ,
   Thanks for your reply.

Light moves slower through transparent denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted. This process causes a time delay in the light's movement

   This is a common explanation and it's good enough for school.  Unfortunately, I don't think it fully explains what is observed.  Absorption and re-emission by atoms should alter the direction of travel of the light.

   I'm concerned that this is hijacking the original post.  The OP should feel free to shout stop at any time.

Best wishes to everyone.

[yor_on]

I don't think of it as 'propagating' as much as it follows those laws we define, and the properties we define to it of course. It's very abstract light and nothing I can hold in my hand. One description is the one in where it becomes a emanation in a field. I think Professor Strassler uses that analogue too, although describing the Higgs field and Higgs boson? Thinking of it that way, and then thinking of the reason given to why it has a 'recoil' it moves it into a very abstract sphere. The law of conservation of momentum

https://www.nuclear-power.com/laws-of-conservation/law-of-conservation-of-momentum/

https://profmattstrassler.com/

and https://scienceblogs.com/builtonfacts/2010/04/21/laser-rifle-recoil

and this law of momentum is a observed fact, but why it must exist moves it away from practical experiments into something abstract.