Final answer:
The vertex distance is the space between the lens and cornea, which can be adjusted for comfort and vision quality. Clear vision is maintained at different object distances through the eye's accommodation process, which changes the lens' power and focal length.
Step-by-step explanation:
When someone mentions that their eyelashes are hitting the back of the lens, they might be referring to the uncomfortable sensation of eyelashes touching the lenses of their glasses. This isn't directly related to vertex distance; however, it highlights the importance of proper eyeglass fitting, which can include adjusting the vertex distance. The vertex distance is the space between the back of the eyeglass lens and the front of the cornea. For comfort and optimal vision, this distance can be adjusted by an optician.
In terms of the Physics of the Eye, clear vision is achieved when the image distance (∂i) is equal to the lens-to-retina distance, which is a constant 2.00 cm in a normal eye. This constancy is maintained for objects at various distances through the process of accommodation, where the eye alters the power and focal length of the lens using ciliary muscles.
The eye's ability to change the focal length and thus the power of the lens to focus on objects at different distances is critical. For objects close to the eye, the lens becomes thicker to increase the refractive power, which allows the image to form precisely on the retina. For distant objects, the lens flattens, decreasing its power. This adaptability ensures that regardless of whether an object is near or far, the image that forms on the retina will be clear and sharp.