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If a shape is said to be concave, it means that it is possible to have two points that to connect, need a straight line segment that must cross outside of the shape. An example would be a cross....
It's also called , a Diverging Lens .It is thinnest in the centre and spreads light out , so it gives a diminished image .
it is a telescope
A concave curve is a curve with negative second derivative. f''(x)<0.