Frances Ashcroft

“Almost all divers wear goggles or a face-mask as without them the eyes are unable to focus underwater and everything appears blurred. This is because when a light ray passes from one medium to another – in this case from air (or water) into the eye – it is bent (refracted). This property is used to help focus the light rays on the layer of light-sensitive cells, known as the retina, at the back of the eye. The extent to which a light ray is bent at the surface of the eye is very much less in water than in air, which makes it impossible to focus the image on the retina. Maintaining an air space next to the eye, by wearing goggles or a face-mask, obviates the problem. But because the light rays will now be refracted by the glass/water interface of the mask, objects appear some 30 per cent larger and closer underwater than they do in air. It may be useful to remember this when listening to divers’ tales of giant sharks.”
― Frances Ashcroft, Life at the Extremes