The power of a lens is a measure of its ability to bend light. It is calculated using the equation P = 1/f, where P represents the power of the lens in diopters (D) and f represents the focal length of the lens in meters (m).

In this question, the power of the lens is given as -2 D. Since power is inversely proportional to focal length, a negative power indicates a concave lens, which causes light rays to diverge.

To find the focal length, we can rearrange the equation to solve for f:

f = 1/P

Substituting the given power (-2 D) into the equation:

f = 1/(-2)

f = -0.5 m

Thus, the focal length of the lens is -0.5 meters.

Option 4 in the question, which states that the focal length is 0.5 meters, is incorrect. The correct answer should be "Alert - correct answer should be -0.5 m."