Refraction is the change in the direction of light when it passes from one medium to another. The working of a lens is based on the refraction of light when it passes through it. The light rays are refracted after passing through the lens. It is of two types, convex lens and concave lens. Refraction of light is the bending of light when it travels from one transparent medium to the other. Light travels at a different speed in different optical media. Refractive index is the refracting ability of a medium. Higher the refractive index denser the medium.
A lens is a transparent refracting medium bounded by two curved surfaces which are generally spherical. A lens is a transparent optical object comprising of one or two curved surfaces that refract light. A lens might either diverge or converge rays of light falling on it, thus forming an image. The concept of refraction of light is used by the lens to form an image.
The most common application of lens is in optical devices such as telescopes and microscopes.
The point on which all parallel rays of light converge is the focal point of a convex lens. The distance between the centre of a convex lens and the focal point of a lens is the focal length. The focal length of a convex lens can be determined by the distant object method.
The point where light rays parallel to the principal axis appear to diverge from after passing through the concave lens is the focal point of a concave lens. The distance between the concave lens to the focal point is the focal length of the concave lens. The focal point of a concave lens is virtual as light does not pass through the point.