Normally, light is focused by the cornea and lens to form a sharp image on the retina.  Long- sightedness occurs when the eyeball is slightly too short, so that objects are in focus behind the retina at the back of the eye.  

This may result in blurred vision when looking in the distance and particularly when looking at near objects and reading.  When we are young, we overcome long-sightedness by bulging out the lens in the eye.   

However, as we get older, the lens gets harder and can no longer bulge out enough.  Long-sightedness can be corrected by spectacles or contact lenses.