 Light rays undergo refraction at the surfaces of each element, but travel in straight lines within the lens.Since the design space of conventional optics is limited to a combination of refractive index and surface structure, correcting for aberrations (for example through the use of achromatic or diffractive optics) leads to large, heavy, complex designs, and/or greater losses, lower image quality, and manufacturing difficulties.Gradient index lenses (or GRIN lenses) as the name implies, are optical elements whose index of refraction varies within the lens.