Lightpipe

A lightpipe is, appropriately, a conductor for light.

Lightpipes used in Transformers toys are made from clear plastic. Light entering the plastic is partly reflected from its polished outer surfaces, and travels through the plastic, to be diffused and emitted elsewhere.

Often such lightpipes have been built into the toys' heads, leaving the clear plastic exposed for the figure's eyes, and in a less noticeable area such as the back or top of the head. Ambient light is conducted to the eyes, lighting them up against the darker face, and the figure's eyes appear to glow.

This effect is also used in toys such as Laser Optimus Prime in which a light source hidden inside part of the toy internally illuminates a clear-plastic weapon. Gnarly!

The use of passive lightpipes in figures' eyes dates back to the European Generation 1 toyline, first utilized by the Turbomasters and Predators.

From Teletraan I: The Transformers Wiki, a Wikia wiki.