concave
US /ˈkɑːn.keɪv/
UK /ˈkɑːn.keɪv/

1.
konkav, hohl
having an outline or surface that curves inward like the interior of a circle or sphere
:
•
The spoon has a concave shape.
Der Löffel hat eine konkave Form.
•
The lens of the telescope is concave.
Die Linse des Teleskops ist konkav.