Focal Length – The distance between the pole P of the concave mirror and the focus F is the focal length of the concave mirror. Thus, the focal length of a concave mirror can be estimated by obtaining a ‘Real image’ of a distant object at its focus, as shown in the figure.

Concave mirrors form both real and virtual images. When the concave mirror is placed very close to the object, a virtual and magnified image is obtained and if we increase the distance between the object and the mirror, the size of the image reduces and real images are formed.

The focal point (F) of a concave mirror is the point at which a parallel beam of light is “focussed” after reflection in the mirror. The centre of curvature (C) is the centre of the circle (sphere) of which the mirror is an arc. The focal length (f) and radius of curvature (R) are defined in the diagram at the right.

10 cm is the focal length of the concave mirror.

Since, object is always placed in front of the mirror hence the sign of object is taken as negative. Since, the centre of curvature and focus lie in front of the concave mirror, so signs of radius of curvature and focal length are taken as negative in the case of concave mirror.

object distance is always negative for all the lens & mirrors. image distance depends on object distance in case of concave mirror and convex lens. ‘v’ is always positive for convex mirror & concave lens.

A concave lens is a lens that possesses at least one surface that curves inwards. It is a diverging lens, meaning that it spreads out light rays that have been refracted through it. A concave lens is thinner at its centre than at its edges, and is used to correct short-sightedness (myopia).

Concave mirror is used by dentist, solar furnace, reflector of a torch, etc. Convex mirror is used in rear view mirrors.