Any non-zero ior value now turns refraction on. In addition to turning refraction on or off, the old refraction keyword was followed by a float value from 0.0 to 1.0. Values in between 0.0 and 1.0 would 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. Note also that neither the ior nor refraction keywords cause the object to be transparent.
Transparency only occurs if there is a non-zero filter or transmit value in the color.
The refraction and ior keywords were original specified in finish but are now properly specified in interior . They are accepted in finish for backward compatibility and generate a warning message.