Myopia (short sightedness) mnemonic, "Cave May Be Long"
CAVE - We use conCAVE lens
MaY - in MYopia
BE - BEfore image is formed before retina
LONG - eyeball eLONGate
When eye become Myopic it is acting as converging lens (+) convex lens (bend the rays coming from object the rays towards centre) more power more convergence more bending so we use diverging lens which is a concave lens to correct or to neutralise the bending or converging.
Concave lens negative (-ve) power.
Another study link! http://medicowesome.blogspot.ae/2013/10/myopia-and-hypermetropia-mnemonic.html