Refraction of parallel ray:
A parallel ray converges at focus of a convex lens and diverges from the focus of a concave lens.
Refraction of ray passing through the Principal focus:
A ray passing through principal focus emerges parallel to the principal axis after refraction from a convex lens.
A ray passing through the principal focus emerges parallel to the principal axis after diverging from a concave lens.
Ray passing through the optical centre of convex and concave lens emerges in same direction without any deviation.
Converging lens: A convex lens is known as converging lens because parallel rays converge at its focus.
Diverging lens: A concave lens is known as diverging lens because parallel rays appear to diverge from the focus; after refraction.
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