Two spherical transparent surfaces are combined to create spherical lenses.
In general, there are two types of spherical lenses.
Concave lenses are formed by uniting two spherical surfaces that curve inward, whereas convex lenses are made by joining two spherical surfaces that bulge outward.
Concave lens:
Concave lenses are narrower throughout the middle and thicker at the corners.
It diverges the incident rays away from the principal axis.
A concave lens has a negative () focal length.
It is used to correct myopic refractive error.
It is used as spy holes in outdoors, telescopes, etc.
Convex lens:
A convex lens is broader in the middle and narrower at its periphery or margins.
It converges the incident rays towards the principal axis.
A convex lens has a positive () focal length.
It is used to correct hypermetropic refractive error.
It is used in cameras, projectors, magnifying lenses, etc.
An illustrative diagram of spherical mirrors is shown below.
