Chapter 18, Problem 75A

To determine

Focal length for lens of eye.

Answer to Problem 75A

  $f=56.25\text{cm}$

Explanation of Solution

Given:

Distance of book from eye, $u=-25\text{cm}$

Distance of image of book from eye $v=-45\text{cm}$

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,

  $\begin{array}{c}\frac{1}{f}=\frac{1}{-45}-\frac{1}{-25}\\ \frac{1}{f}=\frac{1}{25}-\frac{1}{45}\\ =\frac{45-25}{25\times 45}\\ =\frac{20}{1125}\\ f=\frac{1125}{20}=56.25\text{cm}\end{array}$

Conclusion:

Therefore, the focal length in eyeglasses is $f=56.25\text{cm}$ .