If a scuba diver is underwater and looks up, he or she will see a round "hole" directly above him or her at the surface of the water. The rest of the surface will look like a mirror. Explain this phenomenon.
The phenomenon you are referring to is known as the "Snell's window" or "tropical eye". It occurs due to the bending of light when it travels from one medium (water) to another medium (air) with a different refractive index.
When light enters water from the air, it slows down and changes direction due to the increase in density. This change in direction is called refraction. The amount of bending depends on the angle at which the light strikes the water's surface, which is known as the angle of incidence.
When a scuba diver looks up from underwater, they are essentially looking at the surface of the water from a lower angle. As a result, the light rays that reach their eyes have a higher angle of incidence as they pass through the water-air boundary.
Now, Snell's law describes the relationship between the angle of incidence and the angle of refraction. According to Snell's law, when the angle of incidence increases at the water-air boundary, the angle of refraction increases as well. This means that the light rays coming from the water's surface are bent more significantly.
The particular angle at which the light rays from the water's surface refract is dictated by Snell's law. At a certain specific angle, called the critical angle, the light rays are bent to graze along the boundary between the water and air. This causes a phenomenon where a circular "hole" appears above the observer when they look up from underwater.
This circular opening is Snell's window, and it appears round because only the light rays within that specific grazing angle can enter the diver's eye. Beyond that angle, the light gets reflected back into the water, giving the rest of the surface a mirrored appearance.
It's important to note that the size and clarity of Snell's window depend on various factors like the angle of observation, water clarity, and the height of the observer from the surface. So, the phenomenon might not be equally noticeable in all cases.
In summary, when a scuba diver looks up from underwater, they see a round "hole" above them due to Snell's law, where only the light rays within a specific angle of incidence can pass through the water-air boundary and reach the diver's eye.