Chapter-9|Ray Optics and Optical Instrument|NCERT 12th Physics:
Lens Maker's Formula:
With the help of this formula, we can design a lens of desired focal length by adjusting R₁ and R₂.
Derivation:
When a Ray of light from object O incident at first surface it bends towards the normal and if we extend it, then it meet at point I', but due to second surface it bends again away from normal and final image formed at point I, that means I' acts as virtual object for second surface.
The above equation is called lens or thin lens formula.
Note:
