Once an object enters orbit, what keeps it moving sideways?

It doesn't NEED anything to keep it moving 'sideways'.  

Remember Newton's first law:  An object maintains constant uniform motion UNLESS there's a force to make it slow down or turn.  

In space, gravity is usually the only force acting on anything, and gravity acts toward the other object, not sideways.

snell's law

refractve index = sin incidence / sin refraction