Practice question

Given: A concave lens of focal length 25 cm forms an image 10 cm from the lens. What is the object distance? These values define the system as per NCERT data. Formula: Focal length: f = -25 cm (concave lens). This is the standard NCERT relation for this phenomenon. Substitution & Calculation: Image distance: v = -10 cm (virtual image). Lens formula: 1/v - 1/u = 1/f . 1/-10 - 1/u = 1/-25 Rightarrow 1/u = 1/-10 - 1/-25 = -5 + 2/50 = -3/50 . u = -50/3 approx -16.67 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.