Mike FollowsSutton Coldfield, West Midlands, UK Neither water nor air is normally visible because they are largely transparent. What we actually see are boundaries between different media or particles suspended within them. A fish can see the water’s surface from below, just as we can see it from above, because light is reflected and refracted where water meets air.
This is due to the difference in refractive index between the two media. The effect is most noticeable when the surface is rippled, as the changing angles produce shifting reflections. The same principle applies to air bubbles in water or droplets of water in air: it is the boundary between the two media that is visible, not the medium itself.
When viewed close to perpendicular to the surface, a perfectly calm water surface can become almost invisible, allowing you to look directly into the other medium, almost as if you were immersed in it. Refraction also produces “Snell’s window”: rather than seeing the entire world above the surface, a fish sees it compressed into a circular window about 97 degrees across. Outside this window, the surface acts as a mirror because of total internal reflection, reflecting the underwater world back to the fish.
Someone standing above the surface outside this window wouldn’t be directly visible, a fact often exploited by anglers. A fish can see the water’s surface from below because light is reflected and refracted where water meets air Similarly, when you are underwater wearing a mask or goggles, you can’t really see the water itself, just as you can’t see the air on land. You know you are underwater because of the different environment: buoyancy, the altered landscape and the underwater flora and fauna.
Visibility underwater is often poorer than in air because suspended particles scatter light. Even in exceptionally clear water, light is gradually absorbed with depth. Longer wavelengths, such as red light, are absorbed first, leaving mainly blue light to penetrate to greater depths, which is why the underwater world becomes increasingly blue.
So, in much the same way that we don’t normally see the air around us, fish don’t normally see the water around them. Instead, they see objects in the water and the boundaries where water meets something else, particularly the water’s surface. To answer this question – or ask a new one – email [email protected].
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