What is Index of Refraction?

The ability of a lens to bring light to a focus depends on the curvature of its surfaces, and a property of glass called index of refraction.

In optics the refractive index or index of refraction n of a substance (optical medium) is a dimensionless number that describes how light, or any other radiation, propagates through that medium. It is defined as

n = {c}/{v}

where {c} is the speed of light in vacuum and {v} is the speed of light in the substance. For example, the refractive index of water is 1.33, meaning that light travels 1.33 times as fast in vacuum as it does in water. (court. Wikipedia)

Thus, glass with a higher index bends light more strongly than glass with a lower index. Higher index glass allows gentler curves on the lens surfaces for a given focusing power. This is useful in scope design because gentler lens curves don’t force the rays of light to bend too sharply, which would give rise to unwanted defects (aberrations) in the image.