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What is principal focus of alum

Answer»

When rays of light are INCIDENT on a lens or a mirror, they refract/reflect to a certain point on the principal axis. This point is CALLED the FOCUS of the lens/mirror.

This point can be anywhere on the principal axis, depending on how the rays are initially travelling.

 

Principal focus is a special CASE of focus wherein the rays are travelling parallel to the lens/mirror and then refract/reflect to a particular point on the axis.

Generally the rays are parallel if the object is at INFINITY.



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