  We derive the paraxial meridional ray tracing equations from the unified
reflection-refraction law using geometric algebra. This unified law states that
the normal vector to the interface is a rotation of the incident ray or of the
refracted ray or of the reflected ray by an angle equal to the angle of
incidence or of refraction. We obtain the finite meridional ray tracing
equations by simply equating the arguments of the exponential rotation
operators. We then derive the paraxial limits of these equations with the help
of sign function identities. We show that by embedding the sign functions in
the ray tracing equations, we explicitly declare our chosen sign conventions in
symbols and not in prose.
