# Principal Focus And Focal Length Of A Concave Lens

## Principal focus

The principal focus of a concave mirror is the point on its principal axis, to which, after reflection from the concave mirror, all parallel light rays will converge. When a object is placed at an infinite distance from the mirror, the rays of light emerging from the object will be parallel to the principal axis, as shown in the figure. After reflection from the concave mirror, the rays will converge to the focus of the mirror, which is indicated by point F in the figure.

### Focal length

A focal length of the concave mirror is the distance between the pole of the mirror denoted by point P in the image to the focal point of the mirror. Hence it is the distance between points P and F.

Note: The value is taken negatively for the concave mirror, whose curvature radius lies in front of the concave mirror. So the mirror's focal length is according to the convention of a cartesian coordinate system. We need to know not to mix the center of curvature and the focal point of the mirror because the center of the curvature is twice the mirror's focal length.

### How to find the focal length of a concave mirror?

• The spherical mirror whose reflecting surface is about to curve inwards and follows the laws of reflection of light is called a concave mirror.
• The light rays that come from a distant object can be considered to be parallel to each other.
• If the image formed is inverted, real, and small in size, then the parallel light rays meet a point in the front of the mirror.
• If the image formed by the convex lens is real and can also be obtained on the screen.
• f represents the difference between the principal axis P and the focus F of the concave mirror.