Primary and secondary focus of mirror?

Question

Please tell me about the Primary and secondary focus of spherical mirror.

Answer 1

If the aperture( surface area) of the mirror is large, rays incident parallel to the axis at various points on the mirror cut the axis at DIFFERENT points after reflection. we may call these secondary. Only if the aperture is small they approximately cut thru a single focal point after reflection and this point is called the principal focus.Generally large aperture mirrors suffer from the defect called spherical aberration giving blurred and distorted images.

Answer 2

When rays parallel to the principal axix of the mirror falls on the mirror the reflected light is focused at a point on the axix itself- this is primary focus.
When parallel rays at an angle to the principal axis falls on the mirror reflected rays focus on points slightly away from the axis on the same plane. This is called secondary focus.

Answer 3

when u attach a concave lens and silver it on one of it's sides u get a primary focus on the oppositeof the silvering and a econdary focus on the other side of the convex lens