Final answer:
The focal length of a lens will typically increase when it is submerged in water due to the change in refractive index of the surrounding medium, altering the way light is refracted through the lens.
Step-by-step explanation:
The focal length of a lens is dependent on the refractive index of the material from which the lens is made and the refractive index of the medium surrounding the lens. When a lens is submerged in water, the refractive index of the surrounding medium changes because water has a different refractive index than air. Using the lens maker's equation, one can determine that the focal length will change, typically increasing, as the refractive index difference between the lens material and water is smaller than that between the lens material and air.
This change is due to Snell's Law and the principles of refraction. Light entering a medium with a higher refractive index slows down and bends toward the normal line, altering the convergence or divergence of the rays and thus, affecting the focal length. For most lenses, the focal length will increase when submerged in water, implying that the lens will focus parallel rays of light further away from itself than it would in air.
Experimentation with a water bottle lens, as suggested, will help illustrate these concepts and provide a practical understanding of how the depth of water affects the focal length due to its shape acting as a lens.