Question

Presbyopia is the tendency to gradually become far-sighted (hyperopic) as you age. If you have normal vision when you are young, you have a near point of 25 cm. A. If the distance between your eye's lens and retina is 1.71 cm, what is the focal length of your eye's lens when you look at an object at your near point? f = cm B. As you get older, suppose that the near point of your eye increases to 46 cm. What is the focal length of your eye's lens when you look at an object at your near point now? f = cm C. With your near point at 46 cm, what is the focal length of the corrective lens (placed directly in front of your eye's lens) which you would need to look at an object that is 25 cm in front of your eye? f = cm D. As you continue to age, the corrective lenses will no longer be sufficient to allow you to see an object that is 25 cm in front of your eye. When you are wearing your corrective lenses, suppose that you can now see objects only if they are no closer than 36 cm in front of your eye. What is the actual near point of your eye now? N = cm What is the focal length of your eye's lens that corresponds to this new near point?

Answer 1

1.62cm

Step-by-step explanation:

a) object distance do = 25cm

image distance di = 1.71cm

we have the relation

1/f = 1/di +1/do

plugging in the values we get

f = 1.6cm

b) di = 1.71cm

do = 46cm

that gives

f = 1.65cm

C) do = 25cm

di =46 cm

plugging in the values in the above relation we get

f = 16.2cm

D) do = 36cm

f = 16.2cm

plugging in the equation

1/di = 1/f -1/do

plugging in the values we get

di = 29.4 cm

the new near point is 29.4cm

taking do = 29.4cm

di = 1.71 cm

we get focal length of eye as

f = 1.62cm