A person needs a lens of power –4.5 D for correction of her vision. (a) What kind of defect in vision is she suffering from? (b) What is the focal length of the corrective lens? (c) What is the nature of the corrective lens?

Answer:

(a) Myopia

Myopia is the inability of an eye in viewing long distant objects. The image in this case is formed before the retina. For every myopic eye, there exists a far point beyond which a clear image cannot be seen. The short-sightedness is corrected by using a concave lens which diverges and shifts the image to the retina.

(b) P = 1/f

Where

f is the focal length in meters.

P = -4.5 D

f = 1/P

f = 1/4.5D

f = 0.22 m = 22 cm

(c) Negative sign of the focal length indicates that is a concave lens.

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