Practice question

Given: A combination of two lenses in contact has a convex lens ( f = 40 cm ) and a concave lens ( f = 40 cm ). What is the power of the combination? These values define the system as per NCERT data. Formula: f_1 = 40 cm = 0.4 m, f_2 = -40 cm = -0.4 m. This is the standard NCERT relation for this phenomenon. Substitution & Calculation: Power: P = P_1 + P_2 = 1/f_1 + 1/f_2 . P_1 = 1/0.4 = 2.5 D, P_2 = 1/-0.4 = -2.5 D . P = 2.5 + (-2.5) = 0 D . 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.