How is it that contact lenses can be thinner than bifocales but just as effective at fixing someone's sight?
The power or 1/f of contact or framed glasses for the same person needs to be the same. Power is proportional to (N-1)[1/R1 - 1/R2]. N is the index of refraction of the material. N is not much different for contact or glass lenses. The difference in curvatures, [1/R1 - 1R2], is therefore also about the same
The reasons glass lenses are thinner are twofold:
(1) Glass lenses maintain an edge thickness of about 1/8" to make them less fragile and easier for the frames to hold.
(2) Contact lenses only have to cover the pupil of the eye, which means that their diameter can be as small as 1 cm. Glass lenses need to have at least 3 cm clear aperture because thay are located farther fcrom the eye and need to provide a wide field of view.
(3) The increase of thickness at the center, relative to the edge, is proportional to the square of the aperture size. If the glass lens has (typically) three times the diameter of the contact lens, its thickness change at the center must be nine times larger. Add the increased edge thickness to this and you get a much thicker glass lens.
Contact lenses can be thinner than bifocals but still just as effective at fixing someone's sight due to several factors.
Firstly, the power or 1/f of both contact lenses and bifocals needs to be the same for the same person. Power is calculated using the formula (N-1)[1/R1 - 1/R2], where N is the index of refraction of the lens material. The index of refraction for contact lenses and glass lenses is not much different, so the difference in curvatures, [1/R1 - 1/R2], is about the same.
However, there are a few reasons why glass lenses tend to be thicker than contact lenses:
1. Thickness for strength: Glass lenses are designed to maintain an edge thickness of about 1/8" to make them less fragile and easier for frames to hold. This added thickness increases their overall size compared to contact lenses.
2. Coverage area: Contact lenses only need to cover the pupil of the eye, whereas glass lenses need to have a larger diameter to provide a wider field of view. Contact lenses can have a diameter as small as 1 cm, while glass lenses typically require at least 3 cm of clear aperture. The smaller coverage area of contact lenses allows them to be thinner.
3. Aperture size: The increase in thickness at the center of a lens, relative to the edge, is proportional to the square of the aperture size. Since glass lenses typically have a larger diameter (three times or more) than contact lenses, their central thickness change must be nine times larger. When you add this increased thickness to the already thicker edges, it results in a much thicker glass lens compared to a contact lens.
Overall, the combination of a smaller coverage area, reduced thickness for strength, and smaller aperture size allows contact lenses to be thinner than bifocals while still being just as effective at correcting vision.