A lens is a curved piece of material that refracts light. This refraction of light is used to form an image. The two main types of lenses are convex and concave.
Convex lenses are thicker in the middle, which means that they refract parallel rays of light inwards to a single point, which we call the focal point.
Convex lenses are sometimes called converging lenses because they make the light rays converge (come together).
Concave lenses are thinner in the middle than at the edges. This means that the parallel rays diverge (spread out) as they pass through the lens. Although the rays split up, they come from the focal point on the other side of the lens.
We can figure out the direction that the light rays will be refracted. To do this, trace the virtual lines (green) from the focal point of the lens to the point at which the rays meet the lens. If we extend that line, it shows where the real rays will travel.
The focal point will always sit on the axis passing through the middle of the lens. The distance between the focal point and the lens is called the focal length. A shorter focal length is more powerful, which means it refracts light more intensely.
To make a lens more powerful, we can change its shape, making it more curved. Or we can use a material that can refract light more intensely.