When any object absorbs all colors of light than what colour of the object will be visible to us?

The focal length of a convex lens is 15 cm. Then where should the object placed to see the real image of same size. :

An object is placed in front of the concave lens of focal length 20 cm at distance of infinity. What will be a distance of the virtual image from the lens?

If we want to see our full image in the plane mirror then what should be a minimum height of mirror?

An incident ray of light falling on the plane mirror at an angle of 30° then what will angle between incident ray band reflected ray?