Final answer:
The focal length of a lens changes when it is placed in a different medium due to the dependence on the indices of refraction. A lens can act as a converging lens or diverging lens based on whether the new medium's refractive index is lower or higher, respectively, than the lens material.
Step-by-step explanation:
The focal length of a lens changes when placed in a medium with a different refractive index. According to the lens maker's equation, this is because the focal length is dependent on the indices of refraction of the lens material and the surrounding medium. If the refractive index of the new medium is higher than that of the lens, the lens will behave as a diverging lens, and if the refractive index of the medium is lower, it will behave as a converging lens.
When a lens with an index of refraction n2 is placed in a medium with a refractive index of 1.2, if n2 is greater than 1.2, the lens will act as a converging lens. However, if the lens is placed in a medium with an index of 1.65, and if n2 is less than 1.65, the lens will behave as a diverging lens. These changes occur due to the bending of light rays based on the medium's refractive index relative to that of the lens.