Question

what is focal length and nature of thr corrrctive lens..?

Answer 1

heya here is your answer
It is a negative (diverging) lens with a power of -4 dioptres. Since the unit “dioptre” is defined as the reciprocal of the focal length in metres, the focal length is 0.25 m or 250 mm.
If this is a lens in a pair of glasses, its effect is to take light from objects at infinity (i.e. anything distant) and produce a virtual image of those objects at an apparent distance of 250 mm. Objects that are closer than infinity will appear closer than 250 mm. Thus, this is a corrective lens for someone who is sufficiently nearsighted that their far focus limit is 250 mm
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