Final answer:
A lens forms an image through the phenomenon of refraction, which is the bending of light as it passes through a transparent medium, focusing parallel rays to a point called the focus.
Step-by-step explanation:
Formation of an Image by a Lens
A lens forms an image primarily due to the phenomenon of refraction. Refraction occurs when light enters a new transparent medium, such as glass or the crystalline lens of the eye, and is bent towards or away from the line perpendicular to the boundary surface. This bending of light is responsible for the convergence of parallel rays to a single point known as the focus, where an image of the light source is created. The focal length of the lens is the distance from the lens to where these light rays converge. For instance, in a camera or the human eye, lenses are used to form an image on a sensor or the retina, respectively. Ray tracing for thin lenses and equations that describe image formation quantitatively can further illustrate how lenses produce images. This concept not only applies to lenses in devices but is also a key function of our eyes. Whether for enlarging, shrinking, or inverting images, the lens's curvature and its refractive power are critical for proper image formation.