Which part of the eye is a curved piece of glass or plastic that helps to focus light?

The lens of the eye is a transparent, flexible structure located behind the iris and pupil. Its primary function is to focus light rays onto the retina, which is essential for clear vision. The lens achieves this focusing through a process called accommodation, where it changes shape depending on the distance of the object being viewed.

When you're looking at something close, the lens becomes thicker to bend the light more sharply, while for distant objects, the lens flattens out. This ability to precisely focus light allows the brain to interpret the images accurately.

The other parts mentioned serve different functions; for example, the pupil is the opening that allows light to enter the eye, while the iris controls the size of the pupil and hence the amount of light that enters. The optic nerve, on the other hand, transmits signals from the retina to the brain, enabling vision but does not participate in focusing light. Therefore, the lens is the component specifically responsible for focusing light, making it the correct answer.