Concave lens
Explanation
Lenses are transparent optical devices that refract light. They are primarily classified based on their effect on parallel rays of light incident upon them. A lens that bends rays inward toward a focal point is a converging lens, while a lens that bends rays outward away from the principal axis is a diverging lens.
Option Analysis:
- Convex lens is Incorrect: A convex lens is thicker at the center than at the edges. It refracts parallel light rays inward so that they meet at a focal point. Therefore, it is classified as a converging lens.
- Concave lens is Correct: A concave lens is thinner at the center than at the edges. When parallel light rays pass through it, they are refracted outwards, appearing to diverge from a virtual focus. Therefore, it is classified as a diverging lens.
- Concave mirror is Incorrect: A concave mirror is a reflective surface, not a lens. Functionally, a concave mirror reflects parallel rays inward to a focus, acting as a converging device, not a diverging one.
- Prism is Incorrect: A prism is a transparent optical element with flat, polished surfaces that refract light. While it deviates and disperses light into its constituent colors, it is not classified as a lens in the context of converging or diverging parallel beams to form images.
Key Takeaway:
In optics, a concave lens is synonymous with a diverging lens because it spreads light rays apart, whereas a convex lens is a converging lens because it focuses light rays to a point.