# Our Objective:

To find the focal length of a concave lens using a convex lens by:

- Lens in contact method
- Lens out of contact method

# The Theory:

## Concave lens

A concave lens is thinner at the center that at the edges. So the light beams passing through the lens are spread out or diverged. Therefore, the concave lens is called a diverging lens. The image formed by a concave lens is virtual and diminished.

Since a concave lens will not produce a real image, a convex lens is used to measure its focal length. There are two methods to find the focal length of a concave lens.

### a) Lens in contact method

When a concave lens of focal length f_{2} is placed coaxially in contact with a convex lens of focal length f_{1 }and if F is the focal length of the combination, then,

Therefore, the focal length of the concave lens,

### b) Lens out of contact method

The real image (I_{1}) formed by the convex lens will act as the virtual object for the concave lens. When concave lens is interposed between the convex lens and the real image I_{1}, the new real image is formed at I_{2}. If u is the distance of the concave lens from the virtual object I_{1} and v is the distance of the concave lens from the real image I_{2}, then the focal length of the given concave lens is,

# Learning outcomes:

- Students list the properties of concave lens.
- Students deduce and apply the lens formula.
- Students identify the two methods used to determine the focal length of concave lens.