Collision of light waves & matter When light or electromagnetic wave hits an obstacle, what happens?  Are the reactions times always negligible?
By reaction I mean all that happens after the hit like reflection for example.
 A: There are several things that can happen, depending on the nature of the obstacle.
The simplest case is if, classically, we think that the obstacle is not too much smaller than the wavelength of the light. Then, the light wave can be reflected, absorbed, or diffracted. 
Most people are familiar with reflection, the light more or less bounces off the obstacle. The reflected light wave will still travel at the speed of light.
Absorption happens when the material absorbs the energy of the light, and changes its form, usually to heat. This happens as the light travels just below the surface (exactly how far depends on how absorbing the material is.).
Diffraction happens because light is a wave, and happens when light passes close to the edge of an obstacle. The wave tends to spread out into the region where we would think the obstacle should cast a shadow. Again, the diffracted light continues to travel at the speed of light.
Finally, if we have an 'obstacle' that isn't really an obstacle, but lets light pass through, then the light will be slowed down by an amount that depends on the refractive index of the material. This is what causes bending of light in lenses, liquids, etc. The speed can change by a large factor -- there are plenty of materials with refractive indicies several times that of air.
The direct answer to your question depends on what you mean by negligible, exactly. What is negligible? The speed of light is very very high (300,000 km/s), so the time required for a reflected beam to complete a return path, or the extra time taken for light to go through a lens or liquid is correspondingly tiny. But if you are looking on very large distances, or with very precise times, then the effects can be seen.
Extra -- the other case, where the obstacle is smaller than the wavelength of the light, and is a single atom or molecule, the answer becomes different, as you have to take into account the atomic structure. But that's a whole course in atomic physics to explain it all.
