From the Lens Maker's formula, 1/f = (μ_lens/μ_medium - 1)(...). If μ_lens = μ_medium, the term in the first parenthesis becomes zero, making 1/f = 0. Therefore, the focal length f becomes infinite, and the lens acts like a plane glass sheet. [cite: 348, 349, 350, 354, 355, 356, 357, 358]