Why can people see better with spectacles?

Normally, the light rays entering the eyes are bundled and refracted by the eye lens in such a way that they converge exactly on one point on the retina at the back of the eyeball. If the refractive power of the lens does not match the length of the eyeball, this ‘focus’ is shifted either in front of or behind the retina, and the vision becomes blurred. 

Spectacle lenses refract the light to the degree necessary to compensate for the refractive errors, and thus the eye lens can converge it exactly on the retina. To do this, the spectacle lenses must be shaped very precisely. This task is performed by opticians.