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Q88 (NDA-I/2021) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Geometrical optics Answer Verified

According to the New Cartesian Sign Convention, which one of the following is correct is respect of the formula 1f=1v+1u1f=1v+1u, where symbols have their usual meanings?

Result
Your answer: —  Â·  Correct: A
Explanation

The formula 1/f = 1/v + 1/u is specifically known as the mirror formula. According to the New Cartesian Sign Convention, this formula expresses the relationship between the object distance (u), image distance (v), and focal length (f) for spherical mirrors [1]. It is valid for all types of spherical mirrors (concave and convex) and for all positions of the object [1]. In contrast, the relationship for spherical lenses is defined by the lens formula, which is expressed as 1/f = 1/v - 1/u [2]. While the New Cartesian Sign Convention is applied to both mirrors and lenses to determine the signs of u, v, and f [4], the specific mathematical form 1/f = 1/v + 1/u is unique to reflection in spherical mirrors . Therefore, it does not apply to lenses, making option 1 the correct choice.

Sources

  1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.2.4 Mirror Formula and Magnification > p. 143
  2. [2] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > 9.3.7 Lens Formula and Magnification > p. 155
  3. [4] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > What you have learnt > p. 159
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