A swimmer sees only hazy contours of objects when he opens his eyes under water, while they are distinct when using a mask. Why?
Inside water, the focal length of the eye-lens increases or the power decreases. This happens because refracting index of water is greater than that of air. Because the focal length increases, the images of objects inside water are formed on the retina; hence, the objects hazy. When masks are used, the eye remains contact with air of the mask and the power the eye-lens remains unaltered. So, they can see objects distinctly with a mask.