if we are given plano-concave lens, which surface of lens should be taken R1 and which one R2. Consider the question- a diverging lens of focal length F is cut into two identical parts, each forming a plano-concave lens. What is the focal length of each part?

Asked by PARUL TRIPATHI | 14th Jan, 2013, 03:23: PM

Expert Answer:

The relation between the focal length, f, and radius of curvatures R 1 and R 2 is given by the lens makers formula. The lens maker formula for concave lens is,


where µ is the refractive index of the material of the lens.

where R1 and R2 are negative for concave surfaces.

 for diverging lens

1/f= ( ?-1)[-1/R -1/R] Thus, f= -R/[2( ?-1)]
For each plano concave lens:

1/f' = ( ?-1)[-1/R -1/?]

Thus    f'= -R/[( ?-1)].

Hence f' =2 f.

The focal length of each half part will be twice the focal length of diverging lens.

Answered by  | 14th Jan, 2013, 07:12: PM

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