A convex lens (thick in the middle) pulls light rays together; a concave lens (thin in the middle) spreads them apart.
Convex lens: parallel rays converge at the principal focus on the far side.
A lens is a transparent material with at least one curved (spherical) surface. A convex lens bulges outward and is thicker in the middle than at the edges — it bends parallel rays inward to meet at a point, so it's also called a converging lens.
A concave lens does the opposite: it's thinner in the middle than at the edges, and it spreads parallel rays apart so they look like they're coming from a point behind the lens — a diverging lens.
Like mirrors, lenses have a principal axis (through both surfaces' centres of curvature), an optical centre (O) at the middle of the lens (a ray through here passes straight without bending), and a principal focus (F) where parallel rays converge (convex) or appear to diverge from (concave). Because light can pass through a lens from either side, every lens actually has two principal foci, one on each side.
Key exam points
Watch it explained
Spherical Lens — Concave & Convex Lens, Chapter 9, Light, Class 10 Science NCERT · NCERT Science