Paraxial Approximation
A small-angle approximation in geometric optics assuming light rays make small angles $θ$ with the optical axis.
Governing formula
sin θ ≈ θ, tan θ ≈ θ, cos θ ≈ 1
SI unit
rad (radians)
In depth
Linearizes trigonometric ray tracing equations, yielding the Gaussian thin lens and mirror formulas ($1/f = 1/v + 1/u$).
Examples in the real world
Using $1/f = 1/v + 1/u$ to calculate image locations for small aperture lenses.