What happens to a plus power lens when it is moved closer to the eye?

When a plus power lens is moved closer to the eye, it feels weaker due to the change in effective focal length from the new position. A plus lens is designed to converge light rays that are coming from a distance, helping individuals with hyperopia (farsightedness) focus on nearby objects. As the lens approaches the eye, the distance between the lens and the retina decreases, effectively shortening the focal length and resulting in a shift in the optical power.

This phenomenon is influenced by the lens's optical principles; as the lens is moved closer, the angle at which light rays enter the lens changes, leading to a reduction in magnification and an overall feeling of decreased strength. Thus, the lens provides less effective magnification for nearby viewing as opposed to its capability when positioned at an optimal distance. Understanding this principle is important for both users of corrective lenses and professionals in vision care to better assess visual acuity and lens effectiveness.