Light and Optics — Prelims Strategy

Prelims Strategy

Begin by identifying what quantity the question supplies and which quantity it asks for. Draw the normal whenever an angle is involved. If an angle is given to the surface, convert it before applying the reflection law or Snell's law. Do not substitute a ray-separation angle for the angle of reflection.

For a stationary refracting boundary, keep frequency fixed. Use the refractive index to relate speeds and wavelengths, and check whether the ray enters a higher-index or lower-index medium. Total internal reflection needs both the correct index direction and an incidence angle above the critical angle; a large angle alone is not enough.

For lenses, write the sign convention before inserting distances. Determine image location from v and orientation from magnification rather than from one sign used for both purposes. Convert centimetres to metres for dioptres. Finally, reject unqualified device claims: a converging lens can form a virtual image for an object inside its focus, and a real optical fibre can guide light while still losing signal energy.