Answer
The power is -8.3D for glasses, -7.1D for contact lenses.
Work Step by Step
The glasses are designed to take an object at infinity and make a virtual image 14 cm away from the eye that the person can focus upon.
The glasses are 0.020 m away from the eye, so the distance of the image from the lens is only 14 cm – 2 cm = 12 cm. The image is virtual, so the image distance is negative.
$$P=\frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}=\frac{1}{\infty}+\frac{1}{-0.12m}=-8.333D\approx-8.3D$$
The contact lenses are designed to take an object at infinity and make a virtual image 14 cm away from the eye that the person can focus upon. Because contact lenses rest right on the eye, the distance of the image from the contact lens is also 14 cm. The image is virtual, so the image distance is negative.
$$P=\frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}=\frac{1}{\infty}+\frac{1}{-0.14m}=-7.143D\approx-7.1D$$