A concave lens is a lens that is thinner in the middle and thicker at the edges, causing light rays to diverge when passing through it.
A convex lens is thicker in the middle and thinner at the edges, causing light rays to converge when passing through it.
Focal length: The focal length of a concave lens is the distance between the center of the lens and its focal point.
Unlike real images, virtual images formed by a concave lens cannot be projected onto a screen. They can only be seen by looking into the lens.