The power of a lens (biconvex) is 1.25  $m^{-1}$ in particular medium. Refractive index of the lens is 1.5 and radii of curvature are 20 cm and 40 cm respectively. The refractive index of surrounding medium : 

The power of a lens in a particular medium is determined using the lens maker's formula:

Formula:

P = μ2/f = (μ1 - μ2) (1/R1 - 1/R2) 

Where:

• P is the power of the lens.
• μ₁ is the refractive index of the lens material (1.5 in this case).
• μ₂ is the refractive index of the surrounding medium.
• R₁ and R₂ are the radii of curvature of the lens surfaces (20 cm and 40 cm respectively).

Steps for Calculation:

1. Substitute the given values into the lens maker's formula.
2. The equation becomes: 
   1.25 = (1.5 - μ2) (1/0.2 + 1/0.4)
3. Calculate the reciprocal of the radii of curvature: 
   1/0.2 = 5, 1/0.4 = 2.5 
   Adding these values: 
   1/0.2 + 1/0.4 = 7.5
4. Substitute this result back into the equation: 
   1.25 = (1.5 - μ2) × 7.5
5. Expand and solve for μ₂: 
   1.25 = 7.5 × 1.5 - 7.5 × μ2 
1.25 = 11.25 - 7.5 × μ2
6. Rearrange to isolate μ₂: 
   7.5 × μ2 = 11.25 - 1.25 
7.5 × μ2 = 10
7. Divide by 7.5: 
   μ2 = 10 / 7.5 
μ2 = 1.33

Conclusion:

The refractive index of the surrounding medium is 1.33, which is approximately the refractive index of water.