Corrective lenses work by bending the light rays before they enter the eye. If you do not have perfect vision you may require additional bending of the light in order to focus the light onto your retina in order to see clearly. Prescriptions are based on how the degree of light rays are bent (refracted). This is measured is diopters. If a higher degree of bending is needed a higher prescription and thicker glass will be required in order to have the light bent at the correct angle so that it lands on the retina.
Prescription of corrective lenses
Prescription Safety Glasses with Progressive Bifocal Lenses. (n.d.). Retrieved March 03, 2016, from http://blog.rx-safety.com/progressive-lenses-in-safety-glasses/
Can I Scuba Dive if I wear Prescription Glasses? (2009). Retrieved March 03, 2016, from https://www.leisurepro.com/blog/scuba-guides/can-i-sc/uba-dive-if-i-wear-prescription-glasses
How Corrective Lenses are Prescribed
What Are Corrective Lenses And How Do They Function? (n.d.). Retrieved March 1, 2016, from http://www.improveeyesighthq.com/what-are-corrective-lenses.html