Our eyes has a __________ lens whose can be changed at will with the action of __________ muscles.

Explanation

To answer the question, we need to fill in the blanks with the appropriate terms related to the human eye and its functioning.

1. **Identify the type of lens in the eye**:
- The human eye contains a lens that helps in focusing light onto the retina. The type of lens that can form real images is a **convex lens**. Therefore, the first blank is filled with **convex**.

2. **Determine what can be changed at will**:
- The lens in the eye can change its focal length, which allows us to focus on objects at different distances. This ability to change the focal length is known as the **power of accommodation**. Thus, the second blank is filled with **focal length**.

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