Question

Which ray is considered in the first step of the derivation for refraction at a spherical surface?

Options

• A ray passing through the centre of curvature and refracting twice

• A marginal ray far from the principal axis

• A paraxial ray from object O incident on the spherical surface

• A ray reflected from the back of the surface

Answer 1

The derivation begins by considering a paraxial ray from object O incident on the spherical surface. Paraxial rays allow the use of the small-angle approximation, which simplifies the refraction relation leading to the standard formula in terms of u, v, and R.