a) Nelene has to put her eyes within 15 cm of the computer screen to read it. Yo

Question

a) Nelene has to put her eyes within 15 cm of the computer screen to read it. You get her some contact lenses. Determine the focal length and pwer of the contact lenses that she needs to be able to see very distant objects in focus.

b) Frank has to put his book 1.5 m from his eyes to read it. Determine the focal length and power of lenses he'd need to be able to read his book comfortably at 25 cm.

c) Draw a circumstance that represents a lens with a negative focal length, a positive object distance, and a negative image distance. Ie: draw and label the object, image, and shape of the lens in the appropriate places.

Explanation / Answer

a)

the object is at infinite

i.e object distance is do = infinite

1/do = 0

using lens equation

1/f = 1/do - 1/di

1/f = 0 = -1/di

focal length is f = -15 cm

power of the lens is p = 100/f = -100/15 = -6.67 diopter

b) given that do = 25 cm

di = -1.5 m = 150 cm

using

1/f = 1/do + 1/di

1/f = (1/25)-(1/150)

f = 30 cm

power p = 100/f = 100/30 = + 3.33 diopter

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