Principal focus of a concave mirror
Rays of light
incident on a concave mirror, parallel to the principal axis, passes through a
fixed point on the principal axis after reflection. This point is the principal
focus (F) of the concave mirror.
Principal focus of a convex mirror
Rays of light
incident on a convex mirror parallel to the principal axis appear to come from
a fixed point on the other side of the mirror. This point is the principal
focus of the convex mirror.
Focal
length (f)
Distance from the pole of a mirror to its
principal focus is the focal length. It is indicated using the letter f.
f= R/2
Focal
plane
Rays of light coming
from infinity, making different angles with the principal axis get focused at
different points. The plane formed by these points is perpendicular to the
principal axis and passes through the principal focus. This plane is the focal
plane.
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