Final answer:
Objects in water appear shallower due to refraction, which bends light as it moves from water to air, making submerged objects look closer to the surface.
Step-by-step explanation:
The phenomenon where objects in water appear at a depth shallower than their actual depth is due to refraction. When light travels between two mediums like air and water, it changes speed, bending in the process. This bending causes the light rays coming from an object submerged in water to change direction as they enter the air, making the object seem closer to the surface. A classic example is when someone's legs appear shorter when wading in a pool, which is an illusion caused by the refraction of light at the water's surface. A ray diagram can demonstrate this effect, showing that rays from the feet bend towards the normal when exiting the water, thus appearing to originate from a shallower depth.
As an illustration, if you were to look at a straight rod partially submerged in water, the submerged part would appear to bend from the air's perspective due to refraction. This effect is more pronounced when the difference in refractive indices is larger, as between air and water. Hence, clear mountain streams would appear shallower, and devices like side-view mirrors in vehicles or the curved surface of a thermometer capitalize on these optical properties for functionality.