Lens in Water Trick in 59 Seconds! 🔥 | JEE Physics
❓ Question A lens having refractive index 1.6 1.6 1.6 has a focal length of 12 cm when it is placed in air . Find the focal length of the same lens when it is placed in water , given that the refractive index of water is 1.28 . 🖼️ Question Image ✍️ Short Solution This is a pure concept-based JEE optics question . 👉 The shape of the lens does NOT change , 👉 Only the surrounding medium changes , 👉 So we compare focal lengths using the lens maker idea in different media . 🔹 Step 1 — Key lens concept (VERY IMPORTANT 🔥)** For a thin lens: 1 f ∝ ( μ lens μ medium − 1 ) Where: μ lens \mu_{\text{lens}} = refractive index of lens material μ medium \mu_{\text{medium}} = refractive index of surrounding medium 📌 Since radius of curvature is same , we compare focal lengths by ratio only . 🔹 Step 2 — Write ratio formula Let: f a f_a = focal length in air f w f_w = focal length in water Then: f w f a = ( μ lens μ air − 1 ) ( μ lens μ water ...