Question

A person uses eyeglasses with a power of -5.5 D. The lenses are 1.8 cm from the person's eye. What is the furthest this person can see clearly to when not wearing their eyeglasses?
A. 2.7 cm
B. 20 cm
C. 28 cm
D. 35 cm
E. 52 cm

Answer 1

Answer:

The correct answer is option B.

Explanation:

$power(P)=\frac{1}{f}$

f = focal length

P = -5.5 D (negative power means that lens is concave)

$f=\frac{1}{P}=\frac{1}{-5.5 D}= -0.1818 m$

1 m = 100 cm

f = -0.1818 m = -0.1818 × 100 cm= -18.18 cm

$\frac{1}{f}=\frac{1}{u}+\frac{1}{v}$

$u=\infty$

$\frac{1}{-18.18 cm}=\frac{1}{\infty }+\frac{1}{v}$

v = -18.18 cm

v = 1.8 cm - far point

far point = 1.8 cm - v =1.8 cm - (-18.18 cm) =  19.98 cm

But closest answer from the given options is option B.Hence, 20 cm is the correct answer.