The commonly used optical expressions like the lens equation are approximations which are only valid for light rays close to the optic axis for which the approximation sinθ ≈ θ is valid. Such rays are called ‘paraxial rays’. We can define it as:
A ray which makes a small angle (θ) to the optical axis of the system and lies close to the axis throughout the system.
Marginal rays are the rays which pass through the maximum aperture of the spherical mirror. While both paraxial and marginal rays are parallel to the principal axis, the basic difference lies in the distance between them and the principal axis.