Two plano-concave lenses of glass of refractive 1.5 have radii of curvature of 20 and 30 cm. They are placed in contact with curved surface towards each other and the space between yhem is filled with a liquid of refractive index 2/3. Find the focal length of the system.

Explanation

To find the focal length of the system consisting of two plano-concave lenses and a liquid in between, we will follow these steps:

### Step 1: Understand the Configuration
The system consists of two plano-concave lenses placed in contact with their curved surfaces facing each other. The space between them is filled with a liquid of refractive index \( \frac{2}{3} \). The refractive index of the glass lenses is \( 1.5 \).

### Step 2: Identify the Radii of Curvature
- For Lens 1 (left lens):
- Radius of curvature \( R_1 = +\infty \) (since it is plano)
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