Practice question

Given: A double convex lens of refractive index 1.55 has radii of curvature 30 cm and -30 cm . 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 standard NCERT relation. Substitution & Calculation: n = 1.55, R_1 = 30 cm, R_2 = -30 cm . 1/f = (1.55 - 1) ( 1/30 - 1/-30 ) = 0.55 ( 1/30 + 1/30 ) = 0.55 × 2/30 = 1.1/30 . f = 30/1.1 approx 27.27 cm . Result: The computed value matches expected outcome and confirms correct choice as per NCERT.