A standard demonstration of refraction is to show water ripples travelling from deeper to shallow water. The frequency remains constant and the wavelength is observed to be shorter and the velocity lower. BUT when the water wave travels from shollow to deeper water the opposite happens and the wave increases its speed. What causes these changes in speed and in particular what's accelerating the wave. What's happened to Newton's Second Law? A common analogy is a car travelling from tarmac to sand which would slow the car but what force would accelerate the car when travelling from sand to tarmac. How would any explanation then be related to the refraction of light?