**eV**and here I will explain the following topic:

# How do You Determine the Focal Length of a Lens?

## Formula

## What do the formula symbols mean?

## Focal length

`$$ f $$`Unit

`$$ \mathrm{m} $$`

For example, if the object is 1 meter away from the lens, so \(g = 1 \, \text{m} \) and the *sharp* image of the object is 2 centimeters away from the lens, so \(b = 0.02 \, \text{m} \). Then the focal length of the lens is:
`
\begin{align}
f &~=~ \frac{ 1 \, \text{m} ~\cdot~ 0.02 \, \text{m}}{ 1 \, \text{m} ~+~ 0.02 \, \text{m} } \\\\
&~=~ 0.02 \, \text{m}
\end{align}
`

Thus, the focal point of the lens used is at a distance of 2 centimeters from the lens.

## Object distance

`$$ g $$`Unit

`$$ \mathrm{m} $$`

## Image distance

`$$ b $$`Unit

`$$ \mathrm{m} $$`

*sharp*image of the object through the lens. You can adjust the sharpness of the image by, for example, changing the distance between the lens and the object.

**Explanation**

## Video

To determine the focal length of a lens, you must create a *sharp* image of an object on a screen - with a lens. Then you measure the distance \( b \) (screen-lens), as well as the distance \( g \) (object-lens) and can then determine the focal length \( f \) with the help of the lens equation:

Or you can focus a *far away* object (\( g ~\rightarrow~ \infty \): \(1/g ~=~ 0\)), then the image, because of Eq. 1

, lies on the screen exactly at the focal length: \( f ~=~ b \).