How Do Similar Triangles Derive the Thin Lens Equation?

[Larrytsai]

kk so i found out the steps involved deriving the thin lens equation, but what i don't get is when you have 2 similar triangles say triangle abd,edf are congruent how does the 2 triangles make it so its di/f-1. so what I am basically asking is can someone thourougly explain to me what's going on in the steps to deriving the thin lens equation thnx.

 

[mjsd]

Remember: the thin lens equations are only approximation to the real world. one way to get these formulas is to study refraction at spherical surfaces using Fermat's principle and small angle approximation, amongst other things. It is not a straight forward derviation. See for example: "Optics" by Hecht