Chapter 18, Problem 101A

To determine

Focal length of lens in eye.

Answer to Problem 101A

  $f=0.189\text{mm}$

Explanation of Solution

Given:

Distance of book from eye, $u=-35\text{cm}$

Distance of retina from eye $v=0.19\text{mm}$

Formula Used:

The lens formula is given as,

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

Where, $f$ is focal length of lens, $u$ is object distance and $v$ is image distance.

Calculation:

On substituting the given values,

  $\begin{array}{c}\frac{1}{f}=\frac{1}{0.19}-\frac{1}{-35}\\ \frac{1}{f}=\frac{1}{35}+\frac{1}{0.19}\\ =\frac{0.19+35}{35\times 0.19}\\ =\frac{35.19}{6.65}\\ f=\frac{6.65}{35.19}=0.189\text{mm}\end{array}$

Conclusion:

Therefore, the focal length in eye is $f=0.189\text{mm}$ .