What is a spherical lens? We have previously learned about spherical mirrors. We learned that the reflecting surface of a spherical mirror was part of a sphere. Similarly, the refracting surface of a spherical lens is part of a sphere. You will note that here we have used the word refracting rather than reflecting. Thus, a lens causes refraction of light.
A spherical lens is of two types- convex and concave.
A convex spherical lens is thicker at the centre and thinner at the edges. Light rays converge on entering a convex lens thus earning the name-converging lens.
A concave spherical lens is thicker at the edges and thinner at the centre. Light rays diverge from a concave lens, hence the name diverging lens.
There are a few terms that one should know about when studying spherical lens.
Optical centre: The central point of the lens
Centre of curvature: Previously, we have learned about the centre of curvature of spherical mirrors. A similar concept works in a spherical lens as well. Each surface of the lens is a part of a sphere. The centre of each sphere is the centre of curvature of the surface of the lens. As a spherical lens has two refracting surfaces, each lens has two centre of curvature- C1 and C2.
Principal Axis of the lens: A line drawn through the centre of curvatures of the spherical lens.
Principal focus (convex lens): A point on the principal axis where light rays parallel to the principal axis converge after passing through the lens.
Principal focus (concave lens): A point on the principal axis from where light rays (initially parallel to the principal axis) appear to diverge after passing through the lens.
Focal length (f): The distance between the principal focus and the optical centre.
Laws of refraction through a spherical lens
- A ray of light parallel to the principal axis after refraction passes through the principal focus on the other side of a convex lens or appears to diverge from the principal focus on the same side of a concave lens.
- A ray appearing to meet at the principal focus of a concave lens or passing through the principal focus of a convex lens, after refraction emerges parallel to the principal axis.
- A ray passing through the optical centre of a concave of or convex lens does not deviate.
Characteristics of image formed by convex spherical lens
Characteristics of image formed by concave spherical lens