Lens (noun)

1. A piece of glass or other transparent substance with curved sides for concentrating or dispersing light rays, used singly (as in a magnifying glass) or with other lenses (as in a telescope).
2. A device in the eye that focuses light onto the retina.
3. A device in a camera that focuses light onto the film or image sensor.

Origin:

Late 16th century: via latin from greek lēns, lentil (from its shape).

Examples:

1. The lens in a camera focuses light onto the film.
2. He wore glasses with thick lenses.
3. The microscope has several lenses that magnify the specimen.
4. The lens in a person's eye helps to focus light and form an image on the retina.
5. She held the magnifying lens over the page to read the small print.