A pair of glasses is designed for a person with a far-point distance of 2.5 m so

Question

A pair of glasses is designed for a person with a far-point distance of 2.5 m so that she can read street signs 26.5 m away. (The far-point distance is the distance from the eye at which you are just able to properly focus a distant object.) (a) If the glasses are to be worn 1.0 cm from her eyes, what is the needed focal length? -2.75 m You are correct. Your receipt no. is 160-3810 Previous Tries (b) Compare this focal length (find the percent difference) to the focal length she would need if she chooses a style of glasses that fit on her face so that the lenses are instead 2.0 cm away from her eyes. 0.365 Submit Answer Incorrect. Tries 4/5 Previous Tries

Explanation / Answer

(A) 1/f = 1/p + 1/q

object distance, p = 26.5 - 0.01 = 26.49 m

image distance, q = - (2.5 - 0.01) = - 2.49 m

1/f = 1/(26.49) + 1/(-2.49)

f = - 2.74834 m

(B) p = 26.5 - 0.02 = 26.48

q = - (2.5 - 0.02) = - 2.48 m  

1/f = 1/26.48 + 1/(-2.48)

f' = - 2.73627 m

delta(f) = f' - f = 0.0121 m

% diff = (0.0121)/(2.74834) x 100

= 0.44 %

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