A lens is a curved transparent device that bends light, like the lens of eyeglasses or a contact lens you’ll need if the natural lens in your eye needs help. Cameras have a lens, too. Smile!
In an eye, the lens — which is behind the iris — helps focus light on the retina so you can see. It's a lot like the lens in a telescope or camera, which also focuses light and helps form images. You can also use the word lens as a metaphor for how you see things. If you look at sports through the lens of medicine, you might study injuries to players. All these lenses help you see or perceive things.