Question

For a normal eye, the far point is at infinity and the near point of distinct vision is about $25cm$ in front of the eye. The cornea of the eye provides a converging power of about $40D$, and the least converging power of the eye lens behind the cornea is about $20D$. From this rough data estimate the range of accommodation (i.e., the range of converging power of the eye lens) of a normal eye.

• A. 10 to 14 D
• B. 20 to 24 D
• C. 28 to 32 D
• D. 14 to 18 D

Answer 1

Answer: (B)

Given, the power of cornea $=40D$ and least converging power of eye lens $=20D$
To observe the objects at infinity, the eye uses its least converging power means power is maximum,
i.e., $=40+20=60D$
The distance between cornea $=$ focal length of eye lens
$f=\frac{100}{P}=\frac{100}{60}=\frac{5}{3}cm$
To focus objects at the near point on the retina
$u=-25cm,v=\frac{5}{3}cm$
Using Lens formula $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1\times 3}{5}+\frac{1}{25}=\frac{15+1}{25}=\frac{16}{25}$
$\Rightarrow f=\frac{25}{16}cm$
Power of lens $=\frac{1}{f}=\frac{100\times 16}{25}=64D$
$\therefore$ Power of eye lens $=64-40=24D$
Thus, the range of accommodation of the eye lens is $20D$ to $24D$.