Practice question

Given: A double convex lens has radii of curvature 30 cm and -30 cm with refractive index 1.6 . What is its focal length? These values define the system as per NCERT data. Formula: Lens maker’s formula: 1/f = (n - 1) ( 1/R_1 - 1/R_2 ). This is the standard NCERT relation for this phenomenon. Substitution & Calculation: n = 1.6, R_1 = 30 cm, R_2 = -30 cm . 1/f = (1.6 - 1) ( 1/30 - 1/-30 ) = 0.6 ( 1/30 + 1/30 ) = 0.6 × 2/30 = 1.2/30 = 1/25 . f = 25 cm . Result: The computed value matches the expected outcome and confirms the correct choice. Units and powers like J kg⁻¹ K⁻¹, m/s², 10⁻⁵ are properly used as per NCERT.