1. What is the strength of a lens which forms an image 24 cm from the lens when

Question

1. What is the strength of a lens which forms an image 24 cm from the lens when the object is 40 cm away? 2.  A person has a far point of 80 cm from his eyes. Write a prescription for the power of the corrective lens. Assume a lens-to-retina distance of 2.0 cm. (Show the work) 1. What is the strength of a lens which forms an image 24 cm from the lens when the object is 40 cm away? 2.  A person has a far point of 80 cm from his eyes. Write a prescription for the power of the corrective lens. Assume a lens-to-retina distance of 2.0 cm. (Show the work)

Explanation / Answer

1. For a lens

v = 24 cm

u = -40 cm

1/f = 1/v + 1/u

1/f = 1/24 - 1/40

1/f = 1/60 cm-1

1/f = 100/60 m-1

P = 1/f = 1.67 D

2.Far point = 80 cm

For this corrective lens, an object which is infinitely far away should form a virtual image at his far point

u = infinity

v = -0.8 m (For virtual image)

1/f = 1/v + 1/u

1/f = -1/0.8 = -5/4 = -1.25 D

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