Aug 5 2014
A focal point is a common point on the principal axis where all the light rays starting from the object converge. The focal point has a spatial extent known as the blur circle. This non-ideal focusing may occur due to aberrations of the imaging optics.
However, a smallest blur circle known as an airy disk is formed in the absence of aberration due to diffraction from the aperture of the optical system.
The focal point is said to be in focus if the light from the point of object is converged to a large extent and out of focus if the light is not converged properly.
Focal Points of Lenses
As light rays can pass through a lens in either direction, a lens has two focal points - one on each side.
A converging lens focuses incident light which is parallel to its optic axis at a point on the other side of the lens, known as the principal focal point.
For a diverging lens, incident light which is parallel to the optic axis appears to diverge from a point on the same side of the lens, known as the principal focal light.
Elliptical mirrors include two focal points. Light passing through one of these points before hitting the mirror is reflected such that it passes through the other point.
Hyperbolic mirrors have two focal points in which the light from one of these points is reflected as if it came from the other point.
Parabolic mirrors reflect a beam of light such that it appears as if the light is radiated from the focal point.
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