Answer:
Without it, we could not survive on Earth. Earth orbits the sun due to the gravity of the sun, which maintains us at a convenient distance from it to enjoy the sun's warmth and light. It keeps the air we need to breathe and our atmosphere in place. Our planet is held together by gravity.
The focal length of the eye-lens system will be exactly 2.0 cm. The lens thickness is altered depending on the distance of the object being viewed so that the focal length is ALWAYS equal to the lens-retina distance. If this is not true fr an individual, they need glasses :)