Concave lens, like any other lens works on the principle of refraction. We can divide a lens into tiny pieces which resemble a triangular prism. When a ray of light falls on it, due to the shape of the lens piece, the light refracts either inwards our outwards. Concave lens is such that it is thicker outside and thinner inside (opposite to that of a convex lens), so the light rays are refracted outwards and thus they diverge.