1 List of United States cities by population. ^ 2  Population projection is that of the entity known since 2016 as Ciudad de MÃ©xico and previously as the Distrito Federal. ^  Population estimate is that of the Toronto census division, which consists solely of the City of Toronto.
Lens (optics) For other uses, see Lens. A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (elements), usually arranged along a common axis.
1 The lens -- Inside the eye, behind the cornea, the lens completes the job of focusing an image at the back of the eye. 2  The lens is suspended by muscles that can change the thickness of the lens to adjust the focus. 3  The iris and pupil -- The iris is a circular arrangement of muscle lying in front of the lens.