### 6.6.4 Refraction

When light passes through a surface either into or out of a dense medium the path of the ray of light is bent. Such bending is called refraction. 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 in the `interior` to turn on refraction and to specify the ior value. For example:

      object { MyObject pigment {Clear } interior { ior 1.5 } }

The default ior value of 1.0 will give no refraction. The index of refraction for air is 1.0, water is 1.33, glass is 1.5 and diamond is 2.4.

Normally transparent or semi-transparent surfaces in POV-Ray do not refract light. Earlier versions of POV-Ray required you to use the `refraction` keyword in the `finish` statement to turn on refraction. This is no longer necessary. 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. 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 or to use the `fade_power` and `fade_distance` keywords. See "Attenuation".

Note: 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 originally specified in `finish` but are now properly specified in `interior`. They are accepted in `finish` for backward compatibility and generate a warning message.