Light – Reflection and Refraction
What the exam tests
Light – Reflection and Refraction tests the laws of reflection, image formation by plane and spherical mirrors, the mirror and lens formulae, sign convention, refraction and refractive index. Numerical questions on focal length, image distance and magnification are common.
Core rules
- Angle of incidence equals angle of reflection; the incident ray, reflected ray and normal lie in one plane.
- Concave mirrors can form real or virtual images; convex mirrors always form virtual, erect, diminished images.
- New Cartesian sign convention: distances measured from the pole; along the incident light is positive, opposite is negative. Heights above the principal axis are positive.
- Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u.
- Refraction: light bends because its speed changes between media. Snell's law: sin i / sin r = n2/n1.
- Refractive index n = c/v. A denser medium has a larger n and a smaller speed.
- Lens formula: 1/v − 1/u = 1/f; magnification m = v/u; power P = 1/f (in metres), unit dioptre.
Worked example
An object is 20 cm in front of a concave mirror of focal length 10 cm. Find the image. Using sign convention, u = −20 cm and f = −10 cm. 1/v = 1/f − 1/u = −1/10 + 1/20 = −1/20, so v = −20 cm. m = −v/u = −(−20)/(−20) = −1. The image is 20 cm in front of the mirror, real, inverted and the same size; the object is at the centre of curvature.
Traps that cost marks
- Mixing the mirror and lens formulae or their magnification signs.
- Putting f positive for a concave mirror.
- Forgetting to convert focal length to metres when calculating power.
- Thinking a convex mirror can form a real image of a real object.