A lens which is thicker at the edges and thinner at the centre is known as concave lens.
This type of lens bends the rays of light outwards..
Concave lens is also known as diverging lens as it diverges the light rays.
It is made up of number of prisms with the base of each prism away from the centre of the lens.
The central part of the lens is a simple parallel sided glass.
The parallel sided glass slab allows the light rays to pass through it without any deviation.
The upper part of the lens deviates the light rays upwards and lower part deviates the rays downwards.
1. Double concave
2. Plano concave