Question

what is the formula used to calculate the focal length of a lens?
a. $f = d_o - d_i$
b. $f=\frac{d_o\times d_i}{d_o - d_i}$
c. $\frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}$
d. $f = d_o + d_i$

Explanation:

Step1: Recall lens formula

The lens formula is a fundamental relation in optics.
$$\frac{1}{f}=\frac{1}{d_{o}}+\frac{1}{d_{i}}$$
where \(f\) is the focal length, \(d_{o}\) is the object distance and \(d_{i}\) is the image distance.

Answer:

C. \( \frac{1}{f}=\frac{1}{d_{o}}+\frac{1}{d_{i}}\)