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Q.Diametre of a plano-convex lens is 6 cm and thickness at the centre is 3 mm.If speed of light in material of lens is 2×10^8m/s the focal length of te lens is? anyone please answer correctly.☺ |
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Answer» ength of plano - convex lens would be 30cmsee figure, a plano-convex lens of diameter 6CM and thickness at the centre is 3mm is shown in figure.from right angled ∆OAB,OA² = AB² + BO²⇒R² = (RADIUS)² + (radius - thickness)²⇒R² = 3² + (R - 0.3)²⇒R² = 9 + R² - 0.6R + 0.09⇒0.6R = 9.009⇒R = 90.09/6 ≈ 15 cmit is given that, speed of light in material of lens is 2 × 10^ 8m/sso, refractive index, μ = 3 × 10^8/2 × 10^8 = 1.5now using FORMULA, 1/f = (μ - 1)[1/R1 - 1/R2]here, R1 = 15CM , R2 = ∞so, 1/f = (1.5 - 1)[1/15 - 1/0]= 0.5/15= 1/30hence, f = 30 cm |
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