You will be given a definition of a task first, then some input of the task.
You will be given a trivia clue, and the category it belongs to. You should answer with the best answer that belongs in the category and is described by the clue. For simplicity, answers should be in all lower cased letters.

Category: PHYSICS 
Clue: Plano-convex, biconvex & concavo-convex are 3 of the types of this optical component
Output:
lens