very basic dbt.

The paraxial approximation is fairly accurate for angles under about 10°, but is inaccurate for larger angles.In geometric optics, the paraxial approximation is an approximation used in ray tracing of light through an optical system (such as a lens).[1]
A paraxial ray is a ray which makes a small angle (θ) to the optical axis of the system, and lies close to the axis throughout the system.[1] Generally, this allows three important approximations (for θ in radians) for calculation of the ray's path:[1]
sin(θ)≈θ
cos(θ)≈1
tan(θ)≈θ

though most of the problems we are doing as part of the syllabus is not at all paraxial na,So y we have to do something that is rong.Ne logic behind.

Neways brother thanx for ur reply,i was thinking paraxial rays for theta tends to zero.

i hope u have done derivation for mirror formula...
u have to use that only...except omit that part where we apply approximation for rays being paraxial

u r right msp.. as u said most problems we do not need paraxial rays condition..

but for small angles whihc u can infer from the information given in the question , u must haev to use it if required to.

Paraxial rays are close to the principal axis and parallel to it. All the incidence on the mirror are paraxial when the radius of the curvature of the mirror is very large and aperture diameter is small.A paraxial ray is a ray that makes a small angle to the optical axis of the system, and lies close to the axis throughout the system. Such rays can be modeled reasonably well by using the paraxial approximation. When discussing ray tracing this definition is often reversed: a "paraxial ray" is then a ray that is modeled using the paraxial approximation, not necessarily a ray that remains close to the axis.

http://en.wikipedia.org/wiki/Paraxial_approximation

http://www.zemax.com/kb/articles/18/1/Understanding-Paraxial-Ray-Tracing/Page1.html