In designing optical lenses, it is often desirable to employ glass with a high index of refraction. The greater the index of refraction, the greater the bending of the light. Since this reduces the necessary curvature of the glass, the lens can be made thinner and lighter. Unfortunately, glass with a high refractive index can also have a high dispersion. By adding radioactive thorium to the glass, a high refractive index (over 1.6) can be achieved while maintaining a low dispersion.