A thin convex lens made of glass with a refractive index of 1.5 has a focal length of 20 centimeters in air. If this lens is completely submerged in a transparent liquid whose refractive index is exactly 1.5, what will be its new focal length?

💡 Solution & Explanation

According to the lens maker formula, focal length depends on the relative refractive index of the lens material with respect to the surrounding medium. When the refractive index of the liquid matches the refractive index of the glass lens, the relative refractive index becomes one, making the optical power zero and the focal length infinite. This means the lens behaves like a flat glass plate and does not converge or diverge light rays. Immersion in media of different refractive indices modifies both the magnitude and sign of a lens focal length.