A designer wants to create a device that spreads light from a small bulb to illuminate a wide area uniformly. Which lens should be used and why?

To understand which optical device is best suited to illuminate a wide area uniformly from a small light bulb, we must analyze the behavior of light rays when they pass through different media and lenses:

1. Plane Glass: A plane glass sheet allows light rays to pass straight through with minimal bending (refraction) due to its parallel surfaces. It does not spread the light over a wider angle than the source's natural spread.
2. Prism: A prism refracts light at specific angles and disperses it into constituent colors, which does not help in uniform illumination.
3. Convex Lens: A convex lens is a converging lens. When light rays from a source pass through a convex lens, they are bent inward toward a focal point, concentrating the light rather than spreading it out.
4. Concave Lens: A concave lens is thinner at the center than at the edges. When parallel or diverging light rays from a small source (like a bulb) pass through a concave lens, they bend outward away from the principal axis. This refraction causes the light rays to diverge (spread out) in all directions, covering a much larger surface area uniformly.

Therefore, a concave lens is the correct choice because it diverges the light rays outward, effectively spreading the light to illuminate a wide area.