Question

A child is using a spectacle with power of -2.5 d.what is meant by the negative sign?

Answer 1

Negative indicates that the child's spectacles has Concave lens as the power of a concave lens is negative. The child has short-sightedness.

Answer 2

-2.5D represents a concave lens of -0.4m of focal length.
The negative sign shows that image is real and inverted.