Fish can see large areas above and below the water as shown in the diagram below. For angles less than the critical angle, they can see above the water. For angles less than the critical angle, they can see below the water.
Total internal reflection makes periscopes and binoculars possible. Both devices contain 45 degree prisms:
Light strikes the first face of each prism at right angles, or nearly so. It meets each internal face at 45 degrees and is reflected through 90 degrees, emerging perpendicular to the original face. It can do this if the refractive index of glass,
The most impressive modern use is in flexible fibre optic cables, which can transmit light signal for thousands of miles. Light is repeatedly reflected from the sides of the optic cable as long as the angle of incidence is greater than the critical angle.
