Explain why are there two principal focus in case of spherical lenses

1.	Explain why are there two principal focus in case of spherical lenses
Answer» Because they are bispherical lenses, i.e they have two spherical sides (concave or canvex) \xa0lens is made up of two such transparent mirrors , hence when we consider its principle focus both of them are considered. also if the lens has only one focus then a ray diagram is not possible as when the incident ray is at infinity then the light rays cannot converge at the focus of the lens( which is before the lens or on its left side) as the lens undergoes refraction and not reflection.

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