Key Concepts
- 1Mirror formula and sign convention?
- 2Lens formula and power?
Important Formulas & Facts
1/v + 1/u = 1/f. New Cartesian: all distances from pole, +ve right of mirror, -ve left. Object distance u always -ve. Concave f is -ve, Convex f is +ve.
1/v - 1/u = 1/f. Power P = 1/f (metres) in dioptres (D). Convex lens: +ve power. Concave lens: -ve power. For combination: P = P₁ + P₂.
Must-Know Questions
Q1A concave mirror produces a real, inverted image of the same size as the object. Where is the object placed?
When an object is placed at the centre of curvature (C) of a concave mirror, the image is formed at C itself. The image is real, inverted, and of the same size as the object.
Q2State Snell's law of refraction.
Snell's Law: The ratio of sine of angle of incidence to the sine of angle of refraction is constant for a given pair of media. Mathematically: sin i / sin r = n₂₁ (refractive index of medium 2 with respect to medium 1). Also written as n₁ sin i = n₂ sin r. This constant ratio is the refractive index.
Q3What is the mirror formula?
The mirror formula is 1/v + 1/u = 1/f, where v = image distance, u = object distance, f = focal length. All distances are measured from the pole of the mirror. Sign convention (New Cartesian) must be followed.
Q4An object is placed at a distance of 30 cm from a concave mirror of focal length 15 cm. Find the position, nature, and magnification of the image.
Given: u = −30 cm (object in front of mirror), f = −15 cm (concave mirror). Using mirror formula: 1/v + 1/u = 1/f 1/v + 1/(−30) = 1/(−15) 1/v = −1/15 + 1/30 = (−2 + 1)/30 = −1/30 v = −30 cm Image is formed at 30 cm in front of mirror (at C). Magnification m = −v/u = −(−30)/(−30) = −1 Nature: Real (v is negative), inverted (m is negative), same size as object (|m| = 1). Image is at centre of curvature.
Q5Define the refractive index of a medium.
The refractive index (n) of a medium is the ratio of the speed of light in vacuum (c) to the speed of light in that medium (v). Formula: n = c/v. A higher refractive index means light travels slower in that medium (optically denser). For example: n(glass) ≈ 1.5, n(water) ≈ 1.33, n(diamond) ≈ 2.42.
Practice Light - Reflection and Refraction
Reinforce what you just revised with practice questions