ICSE Class 10 Physics: Refraction of Light and Lenses — Important Questions with Answers 2026

Short Answer Questions (2-3 Marks)

• Q: Define refraction of light. State two laws of refraction.
  Ans: Refraction is the bending of light when it passes from one transparent medium to another due to change in speed. Laws: (1) The incident ray, refracted ray, and normal at the point of incidence all lie in the same plane. (2) Snell's Law: The ratio of sine of angle of incidence to sine of angle of refraction is constant (sin i / sin r = μ).
• Q: Define refractive index. How is it related to speed of light?
  Ans: Refractive index (μ) of a medium is the ratio of speed of light in vacuum to speed of light in that medium. μ = c/v. It is also equal to sin i / sin r (Snell's law). μ has no unit. For glass: μ ≈ 1.5, for water: μ ≈ 1.33.
• Q: Define critical angle. State two conditions for total internal reflection.
  Ans: Critical angle is the angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°. Conditions for total internal reflection: (1) Light must travel from a denser medium to a rarer medium. (2) Angle of incidence must be greater than the critical angle.
• Q: Distinguish between a convex lens and a concave lens.
  Ans: Convex lens: thick at centre, thin at edges, converges light rays, forms real and virtual images, positive focal length. Concave lens: thin at centre, thick at edges, diverges light rays, always forms virtual, erect, diminished images, negative focal length.
• Q: State the sign convention for lenses.
  Ans: New Cartesian Sign Convention: (1) All distances are measured from the optical centre. (2) Distances in the direction of incident light are positive. (3) Distances against incident light are negative. (4) Heights above principal axis are positive, below are negative. Focal length: positive for convex, negative for concave.

Long Answer / Application Questions (4-6 Marks)

• Q: Define power of a lens. A convex lens has focal length 25 cm. Find its power.
  Ans: Power of a lens is the reciprocal of its focal length in metres. P = 1/f. SI unit: Dioptre (D). Given: f = 25 cm = 0.25 m. P = 1/0.25 = +4 D. Positive sign indicates convex (converging) lens.
• Q: An object is placed 30 cm from a convex lens of focal length 20 cm. Find the image position and magnification.
  Ans: Using lens formula: 1/v - 1/u = 1/f. u = -30 cm, f = +20 cm. 1/v = 1/20 + 1/(-30) = 1/20 - 1/30 = (3-2)/60 = 1/60. v = +60 cm (real image, on other side). Magnification m = v/u = 60/(-30) = -2 (inverted, magnified).
• Q: What is the difference between real image and virtual image?
  Ans: Real image: formed by actual intersection of refracted/reflected rays, can be obtained on a screen, always inverted. Virtual image: formed by apparent intersection of rays (when extended backward), cannot be obtained on a screen, always erect.

Exam Tips for This Chapter

• Revise all definitions and laws from Refraction of Light and Lenses — they are commonly asked as 1-2 mark questions
• Practice diagrams related to Refraction of Light and Lenses — neat labelled diagrams carry 2-3 marks
• For numericals, always show formula → substitution → answer with correct units
• Previous year analysis shows Refraction of Light and Lenses carries 8-12 marks in the board exam