How to measure the refractive index of a lens? how to measure the refractive index of a lens? (a simple method)
I want to find a simple method to  measure the refractive index of a lens.
 A: The lensmaker's equation is 
1/f = (n − 1)( 1/R1 − 1/R2 + (n − 1)d/(n R1 R2)),

where f is the focal length of the lens, R1 is the radius of the first surface, R2 is the radius of the second surface, and n is the refractive index of the lens.  To use this equation to find the refractive index, you need to determine f, R1, and R2.  f is easy: just illuminate with collimated light and see where the focal point is.  R1 and R2 are more difficult.  I would use first-surface reflection: illuminate the lens with a laser pointer whose beam is aligned with the axis of the lens.  Move the laser(or lens) without tilting it, a small measured distance away from the axis and measure the angle of reflection.  Using simple trigonometry, you an determine the radius of curvature for each surface.  Plug the values of f, R1, and R2 in the equation and solve for n.
Another method would be to measure the focal length of the lens when it is immersed in liquids of different known refractive index.  Three such measurements, even if R1 and R2 are not known are sufficient to give you a set of simultaneous equations that can be solved for n.
A: Use a spherometer to measure the radius of curvature of the two lens faces $r_1$ and $r_2$.  
Use any standard method to measure the focal length of the lens $f$.  
Use the lens makers formula $\dfrac 1f = (n-1)(\dfrac {1}{r_1}+ \dfrac {1}{r_2} )$ to find the refractive index $n$ of the lens.
