Plano concave lens( flat concave lens) is a negative len (diverging lens), consisting of a flat surface and an inward-curving concave spherical surface, it's a part of spherical lens. it's core feature is the focal length is negative, causing the light to diverge.
The opposite plano convex lens is positive lens( converging lens).One side if flat while the other side is convex spherical surface, it's also a part of spherical lens. it's core feature is focal length is positive and converges light rays in optical systems.
Bi-concave lens (double concave lens) is a negative lens( diverging lens) with both sides being concave, which commonly used in symmetrical optical paths or inscenarios where a larger divergence angle is required.
Comparison of Plano convex lens and Plano concave lens
Dimension
Plano convex (flat convex)
plano concave( flat concave)
Shape
Middle thick, edge thin
Middle thin, edges thick
Focal Length
Postive f>0
Negative value f<0
Light path
converging…