Our eyes have a convex lens whose focal length can be changed at will by the contraction and relaxation of ciliary muscles.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Structure of the Eye**: The human eye contains a convex lens, which is responsible for focusing light onto the retina at the back of the eye. The position of the lens and the retina is fixed.

2. **Role of the Ciliary Muscles**: The ciliary muscles are connected to the lens of the eye. These muscles can contract or relax, which allows the lens to change its shape.

3. **Changing the Focal Length**: When the ciliary muscles contract, the lens becomes more rounded. This increases the converging power of the lens, thereby reducing its focal length. Conversely, when the ciliary muscles relax, the lens flattens, which increases the focal length.

...