CBSE · Class 10 · Physics · Chapter 9

Light — Reflection and Refraction — Formula Sheet

Board Formulas

10 formulas

1.Laws of Reflection
$∠ i = ∠ r$
Angle of incidence equals angle of reflection; both measured from the normal. Incident ray, reflected ray and normal lie in the same plane.
2.Mirror Formula★
$\frac{1}{v} + \frac{1}{u} = \frac{1}{f}$
$u$ : Object distance from pole (m); negative for real object · $v$ : Image distance from pole (m) · $f$ : Focal length (m); negative for concave, positive for convex mirror
Relates object distance u, image distance v and focal length f of a spherical mirror. Uses sign convention.
3.Magnification (Mirror)★
$m = \frac{h ^{'}}{h} = - \frac{v}{u}$
Ratio of image height to object height. Negative m ⇒ real, inverted image.
4.Focal Length and Radius of Curvature
$f = \frac{R}{2}$
Focal length equals half the radius of curvature (for both spherical mirrors and thin spherical surfaces).
5.Refractive Index
$n_{21} = \frac{n _{2}}{n _{1}} = \frac{s i n i}{s i n r} = \frac{v _{1}}{v _{2}}$
$n$ : Refractive index (dimensionless) · $i, r$ : Angles of incidence and refraction · $v_{1}, v_{2}$ : Speed of light in the two media (m/s)
Snell's law: ratio of sines equals ratio of speeds. n = c/v (absolute).
6.Absolute Refractive Index
$n = \frac{c}{v}$
c = speed of light in vacuum (3 × 10⁸ m/s); v = speed in the medium. Always ≥ 1 for real materials.
7.Lens Formula★
$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$
Note the MINUS sign (differs from mirror formula). u negative for real object; f positive for convex, negative for concave lens.
8.Magnification (Lens)
$m = \frac{h ^{'}}{h} = \frac{v}{u}$
For lens, m = v/u (no minus sign). Positive m ⇒ virtual erect image.
9.Power of a Lens
$P = \frac{1}{f (in metres)}$
Unit: dioptre (D). Positive for converging (convex), negative for diverging (concave) lens.
10.Combination of Lenses in Contact
$P = P_{1} + P_{2} + \dots \frac{1}{f} = \frac{1}{f _{1}} + \frac{1}{f _{2}} + \dots$
Powers add algebraically (with sign). Very common in board numericals.