A __________ is a piece of transparent glass bounded by two spherical surfaces.

Explanation

To answer the question "A __________ is a piece of transparent glass bounded by two spherical surfaces," we can follow these steps:

### Step 1: Understand the Definition
We need to identify what type of optical device is made from transparent glass and has two spherical surfaces.

### Step 2: Identify the Types of Lenses
There are two primary types of lenses that fit this description:
- **Convex Lens**: This lens is formed by two outward-curving spherical surfaces. It is also known as a converging lens because it converges light rays that are parallel to its principal axis.
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