What type of lens would you use to expand a beam of light?

To expand a beam of light, a concave lens is the appropriate choice. Concave lenses are diverging lenses, which means they spread light rays outwards as they pass through the lens. When parallel light rays enter a concave lens, they will diverge and create an apparent focal point behind the lens, giving the effect of expanding the beam.

In contrast, convex lenses, including plano-convex lenses, cause light rays to converge to a point. This convergence would lead to a reduction in the beam's diameter rather than an expansion. Similarly, cylindrical lenses are used to expand light in only one direction, typically for applications requiring specific shapes or patterns of the light beam, rather than uniformly expanding the beam in all directions.

Therefore, for the purpose of expanding a beam of light, a concave lens is the correct choice as it effectively spreads the light rays outward.