**Focal Length and Power of a Lens**

To find the focal length of a lens with a power of -2.0D, we need to understand the relationship between focal length and power.

1. **Power of a Lens**:
The power of a lens is a measure of its ability to converge or diverge light. It is denoted by the symbol 'P' and is measured in diopters (D). The power of a lens is defined as the reciprocal of its focal length in meters.
Mathematically, power (P) = 1/focal length (f).

2. **Focal Length of a Lens**:
The focal length of a lens is the distance between the lens and its focal point. It is the point where parallel rays of light converge (for a convex lens) or appear to diverge (for a concave lens) after passing through the lens.

**Calculating Focal Length from Power**:
To find the focal length of a lens with a given power, we can use the formula:
focal length (f) = 1/power (P).

In this case, the given power is -2.0D. Applying the formula, we can calculate the focal length as follows:

f = 1/P
= 1/-2.0D
= -0.5 meters

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

**Type of Lens**:
Based on the given power (-2.0D), we can determine the type of lens.

1. **Positive Power (Convex Lens)**:
If the power of a lens is positive, it means that the lens is a converging lens or a convex lens. A positive power indicates that the lens can bring parallel rays of light together to a focal point. Convex lenses are thicker at the center and thinner at the edges.

2. **Negative Power (Concave Lens)**:
If the power of a lens is negative, it means that the lens is a diverging lens or a concave lens. A negative power indicates that the lens causes parallel rays of light to spread out. Concave lenses are thinner at the center and thicker at the edges.

In this case, since the power is -2.0D (negative), the lens is a diverging lens or a concave lens.

In summary, the lens in question has a focal length of -0.5 meters and is a concave lens.