Assertion: A convex has a real focus.
Reason: All light rays pass through the focus of a convex lens after refraction.

Dear Student,
A convex lens converge light rays at its focus therefore it is said to produce real focus. While in convex mirrors, the rays diverge but appear to come from a point behind the mirror i.e. the rays of light do not actually meat but virtually appear to meet.The image cannot be taken on a screen. Hence, a convex mirror is said to have a virtual focus.
The assertion provided by you does not clarify whether it is asking about convex lens or convex mirror. So, for convex lens the assertion is correct but for convex mirror assertion is incorrect.
A convex lens is a converging lens. When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus. So, it is correct that all light rays pass through the focus of a convex lens after refraction.
Hence, both assertion (if it is asking about convex lens) and reason are correct and reason is correct explanation of assertion. And if the question is asking about convex mirror than assertion is incorrect but reason is correct and reason is not correct explanation of assertion.

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rincipal or first focus is the point from which incident rays after passing through convex lens, become parallel to the principal axis.

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