Find the focal length of a concavo-convex lens (Positive +ve menisus ) with `R_(1) = 15cm`, and `R_(2) = 25cm`. The refractive index of the lens material `n = 1.5`m
`(1)/(f) = (1.5-1)((1)/(15)-(1)/(25)) = 0.5((10-6)/(150)), :. F = (300)/(4) = 75cm`

Explanation

To find the focal length of a concavo-convex lens using the lensmaker's formula, we can follow these steps:

### Step 1: Identify the parameters
We are given:
- Radius of curvature of the first surface, \( R_1 = 15 \, \text{cm} \)
- Radius of curvature of the second surface, \( R_2 = 25 \, \text{cm} \)
- Refractive index of the lens material, \( n = 1.5 \)

...