Adding refraction 1.0 to the finish statement turns on refraction. Values in between will darken the refracted light in ways that do not correspond to any physical property. Many POV-Ray scenes were created with intermediate refraction values before this bug was discovered so the feature has been maintained. A more appropriate way to reduce the brightness of refracted light is to change the filter or transmit value in the colors specified in the pigment statement. Note also that refraction does not cause the object to be transparent.
Transparency only occurs if there is a non-zero filter or transmit value in the color.
The amount of bending or refracting of light depends upon the density of the material. Air, water, crystal and diamonds all have different densities and thus refract differently. The index of refraction or ior value is used by scientists to describe the relative density of substances. The ior keyword is used in POV-Ray to specify the value. The default ior value of 1.0 will give no refraction.