Final answer:
Objects in water appear shallower due to refraction, which also contributes to underwater injuries from diving in shallow areas with unseen obstacles. Submarines must consider the pressure of water at depth, which plays a role in problems like calculating the force required to open a hatch underwater.
Step-by-step explanation:
Objects in water appear to be at a depth shallower than they actually are because of refraction. This is an optical phenomenon that occurs when light waves change direction as they move from one medium to another, in this case, from water to air. This bending of light makes the position of the object appear higher than where it physically resides.
People sometimes sustain neck and spinal injuries when diving into unfamiliar ponds or waters because the actual depth may be less than they perceive due to refraction, leading to diving in shallow waters, and thus hitting the bottom. Furthermore, unseen rocks or other obstacles may also contribute to such injuries.
In regards to a submarine, when at depth, they face immense pressure from the water above. An example problem involving a stranded submarine requires calculating the force needed to open the hatch using the given parameters, like the hatch size and the air pressure inside. The key property to consider here is the external water pressure at depth, which is a product of the water density, the gravitational constant, and the depth of the hatch below the surface.