What force or acceleration causes the pendulum to speed up on the way down and slow down on the way up

In the attached diagram it is possible to observe as a free-body diagram, the forces acting on the pendulum. One of the Forces that reverberates on this, is the force of gravity, and the same that is responsible for giving the weight value to the mass that hangs on the rope.

If this force acted without having the effects of energy loss by rotation, the pendulum would be in perpetual oscillation. In turn, this gravitational force is the same that is responsible for accelerating and decelerating the pendulum.

speed of light in vacuum is 300,000 km/s

speed of light in other mediums is given by the expression v = c/n, where c is the speed of light in vacuum and n is the refractive index of medium.

for the first medium v = 200,000 km/s

  so, refractive index of medium = 300000/200000 = 1.5

    1.5 refractive index is usually for glass. so first medium is glass.

for the second medium v = 250,00 km/s

so, refractive index of medium = 300000/250000 = 1.2

    1.2 refractive index is usually for water. so second medium is water.

for the third medium v = 0 km/s

so, refractive index of medium = 300000/0 = ∞

    ∞ refractive index is usually for opaque mediums. so third medium is opaque medium.