Doubtify – JEE Ki Taiyaari, Simple Bhasha Mein

❓ Concept 🎬 Focal Length of Lens in Water — Concept in 59 Sec Lens ko air se nikaal kar paani mein daal diya… aur suddenly focal length change ho gaya? 🤔 👉 Isme koi magic nahi — sirf relative refractive index ka game hai 🔥 🖼️ Concept Image ✍️ Short Explanation JEE mein “lens in water” wale questions numerical kam, conceptual zyada hote hain. Agar tum lens maker + relative refractive index samajh gaye, toh answer seconds mein aa jaata hai 😎 🔹 Step 1 — Lens Maker’s Formula (in Air) For a thin lens in air: 1 f air = ( μ lens − 1 ) ( 1 R 1 − 1 R 2 ) Yahaan: μ lens \mu_{\text{lens}} ​ = refractive index of lens w.r.t air R 1 , R 2 R_1, R_2 ​ = radii of curvature 📌 Lens ka shape fixed hota hai — curvatures change nahi hote. 🔹 Step 2 — Lens Placed in Another Medium Agar lens ko kisi aur medium (jaise water) mein rakh diya: 👉 Refractive index relative ho jaata hai: μ relative = μ lens μ medium Modified lens maker formula: 1 f medium = ( ...

❓ 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 ...