The question revolves around the concept of "power of accommodation" in the human eye. Let's explore the options provided and identify the correct one:

1. The cornea provides all the focusing power required for clear vision.

This statement is incorrect. While the cornea does contribute significantly to the eye's focusing power, it does not have the ability to change its shape to focus on near and distant objects. Thus, it cannot be responsible for accommodation.

2. The iris controls the amount of light entering and helps to focus.

Although the iris controls the amount of light entering the eye by adjusting the size of the pupil, it does not help in focusing on objects at different distances. The iris mainly acts like a camera aperture.

3. The retina moves forward and backward to adjust the focus.

This option is incorrect. The retina is a fixed part of the eye, and its movement does not assist in focusing. Instead, it acts as a screen where images are formed.

4. The eye lens changes its focal length by altering its shape through ciliary muscle action.

This is the correct statement. The lens of the eye adjusts its shape to focus on objects at varying distances; this process is known as "accommodation." The ciliary muscles contract and relax to change the curvature of the lens, allowing the eye to focus on both near and distant objects. When looking at nearby objects, the lens becomes thicker to increase its converging power, and for distant objects, it becomes thinner.

Conclusion: The statement that best describes how the power of accommodation enables the human eye to focus on both near and distant objects is: "The eye lens changes its focal length by altering its shape through ciliary muscle action."