Which of the following means that a lens can focus light? a.-di b.+di c.-f d.+f

D. +f

Explanation:

A lens works by refraction: it bends light rays as they pass through it so they change direction. A lens that can focus light is called a convex lens or converging lens, Light rays passes through the lens, refracts and are focused on a single point.

There's a simple measurement that tells you how powerful a lens is and it's known as the focal length. The focal length of a lens is the distance from the center of the lens to the point at which it focuses light rays. The shorter the focal length, the more powerful the lens.

+f means that the light rays coming from one side of a lens are focused on the other side of the lens at point F. In this case light can be focused.

-f means that the light rays coming from one side of a lens after being refracted through the lens appear to be diverging from a single point F' on the same side of the lens.

if the thermos has a vacuum innner, then heat through conv currents and conduction are reduced = vacuum flask

increase masses, decrease separation