Step-by-step guide to Cartesian sign conventions, focal length calculations, and magnification for Class 10 boards.
Light Reflection and Refraction is one of the most important chapters in Class 10 CBSE Science. Mastering ray diagrams and numerical problems ensures top performance in board exams and builds strong Physics fundamentals.
New Cartesian Sign Convention: All distances are measured from the Pole (mirrors) or Optical Center (lenses). Distances measured in direction of incident light are positive; opposite to incident light are negative. Object distance (u) is ALWAYS negative. Concave mirror/lens focal length (f) is negative; Convex mirror/lens focal length (f) is positive.
Mirror Formula: 1/f = 1/v + 1/u. Magnification (m) = -v/u = h'/h. Negative magnification indicates real, inverted image; positive magnification indicates virtual, erect image.
Lens Formula: 1/f = 1/v - 1/u. Magnification (m) = +v/u = h'/h. Power of Lens (P) = 1/f (in meters), measured in Dioptres (D). Dr. Aarzoo Saini conducts numerical practice workshops for Class 10 students at We-Gyaan Classes Roorkee.
Key Takeaways for Students
- Always apply Cartesian sign convention before substituting values in mirror/lens formulas.
- Remember that object distance (u) is always negative.
- Concave focal length is negative; Convex focal length is positive.
- Express focal length in meters when calculating Power of Lens in Dioptres.
Authored by Dr. Aarzoo Saini
Founder & Lead Educator at We-Gyaan Classes Roorkee, with over 20 years of teaching excellence in Science and Chemistry for Board Exams, NEET, JEE, and CUET.