How can we determine the focal length and power of the concave lens required to correct a myopia eye ?

Explanation

Let x be the distance of the actual far point from the eye and hence from the concave lens placed close to the eye. The rays coming from infinity after refraction through the concave lens, appear to come from the far point F.
`:.` object distance `= u = -oo`, image distance `= v = -x`, focal length = f = ?
By lens formula,
`(1)/(f) = (1)/(v) - (1)/(u) = (1)/(-x) - (1)/(-oo) = (1)/(-x) + 0 = (1)/(-x)`
`:.` Required focal length , f = -x.
Required power , `P = (1)/(f) = (1)/(-x)`
The negative sign shows that the correcting lens is a concave lens.