Define the principal focus of a concave mirror.

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The point on the principal axis of a concave mirror where the rays of light coming from infinity or parallel to the principal axis meet after reflection from the mirror is called the principal focus of a concave mirror.

It is denoted by the symbol F.
The principal focus is also referred to as simply focus or focal point.
The principal focus of a concave mirror lies in front of the mirror.
The distance between the pole and the principal focus of the concave mirror is called the focal length.
A line passing through the principal focus and perpendicular to the principal axis on which the rays of light coming from a distant object making different angles with the principal axis converge or diverge is known as a focal plane.
A concave mirror is also known as a convergent mirror.

Location of principal focus of the concave mirror

Location of principal focus of the concave mirror

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Relevant Concepts
Difference between Reflection and Refraction	Laws of Reflection	Mirror Formula Derivation
Difference between Mirror and Lens	Reflection of Light	Refraction by Spherical Lenses
Power of a Lens	The Refractive Index	Concave Lens
Lens Formula and Magnification	Convex Lens	Image Formation by Lenses

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