Briefly describe how you would find the focal length of a concave mirror quickly but approximately.

For determining the focal length of a concave mirror, the object should be placed a considerable distance from the concave mirror such that the image is formed at the focus. This is done by focusing a distant object such as a tree by using a concave mirror. A sharp image of the tree is formed at the focus of the concave mirror. Therefore, the distance of the image from the concave mirror will be equal to the focal length of the concave mirror.