Light — Mirrors, Lenses and the Two Formulae That Solve Both

Why Optics Pays

Optics is the highest-yield chapter in general science: almost every recruitment paper asks one or two questions, and they come from a very small pool of ideas.

The two laws of reflection:

• ›The angle of incidence equals the angle of reflection (∠i = ∠r).
• ›The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.

Both hold for every mirror — plane, concave or convex.

The sign convention decides your answer. Distances are measured from the pole of the mirror, or the optical centre of the lens:

• ›The object is always placed on the left, so u is always negative.
• ›Distances measured along the incident light are positive; against it, negative.
• ›Concave mirror: f is negative. Convex mirror: f is positive.
• ›Convex lens: f is positive. Concave lens: f is negative.

Get the signs right and the arithmetic is trivial. Get them wrong and a correct-looking wrong answer is already waiting in the options.

1. The focal length of a concave mirror whose radius of curvature is 30 cm is:

2. An object is placed 15 cm from a convex lens of focal length 10 cm. The image formed is:

3. The mirror used as a rear-view mirror in vehicles is a:

4. The power of a lens of focal length −50 cm is:

5. When white light passes through a glass prism, the colour that deviates the most is:

6. Myopia, or short-sightedness, is corrected by using:

7. Total internal reflection can occur only when light travels:

8. The image formed by a plane mirror is always: