A farsighted person has a nearpoint of 41 cm from her eyes. She wants glasses th

Question

A farsighted person has a nearpoint of 41 cm from her eyes. She wants glasses that will let her see objects at a distance of only 25 cm from her eyes. Determine the focal length of the glasses needed if the glasses are 2 cm and 3 cm from her eyes. (Remember, a converging lens has a positive focal length and a diverging lens has a negative focal length.) f_2cm =.41 cm f_3cm = 52.388 cm A nearsighted person has a farpoint of 516 cm from his left eye and 633 cm from his right eye. He wants contact lenses that will let him see objects at a great distance. Determine the focal length of the contact lens needed for each eye. (Remember, a converging lens has a positive focal length and a diverging lens has a negative focal length.) f_left = -516 cm f_right = -633 cm

Explanation / Answer

a) A farsighted person has a nearpoint of 41 cm from her eyes. She wants glasses that will let her see objects at a distance of only 25 cm from her eyes. Determine the focal length of the glasses needed if the glasses are 2 cm and 3 cm from her eyes.

To find F2, you need to subtract 2 cm from nearpoint and 25 cm.
1/f = -1/39 + 1/23

f2=56.06 cm


To find F3, you need to subtract 3 cm from nearpoint and 25 cm.
1/f = -1/38 + 1/22

f3=52.25 cm

(b) A nearsighted person has a farpoint of 516 cm from his left eye and 633 cm from his right eye. He wants contact lenses that will let him see objects at a great distance. Determine the focal length of the contact lens needed for each eye.

No subtraction is needed to solve for F:
For the left eye, farpoint = 516cm
1/f = 1/infinity - 1/516
f = -516
:. F(left eye) = - 516cm

For the right eye, farpoint = 633cm
1/f = 1/infinity - 1/633
f = -633
;. F(right eye) = - 633cm

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