The focal length of a 2D lens is 50 cm. The power of a lens is the inverse of the focal length in meters, and the physical unit for lens power is 1/meter, which is called dioptre (D) . Therefore, if the power of a lens is 2D, the focal length of the lens can be calculated as follows:

focal length = 1/power = 1/2D = 0.5 meters = 50 cm

Hence, the focal length of a 2D lens is 50 cm.