If a sheet of glass is placed in the tank, the depth of water in the tank will be shallower over the glass than elsewhere. The speed of a wave in water depends on the depth, so the ripples slow down as they pass over the glass. This causes the wavelength to decrease. If the junction between the deep and shallow water is at an angle to the wavefront, the waves will refract. In the diagram above, the waves can be seen to bend towards the normal. The normal is shown as a dotted line. The dashed line is the direction that the waves would travel if they had not met the angled piece of glass.