10.6 Going Deeper: Enrichment & Exam Preparation

Light at Work Around You

Mirrors and Lenses in Daily Life A headlight reflector (concave mirror) throws a beam down the road; a side-view mirror (convex) shows wide traffic behind; a magnifying glass and your spectacles (convex/concave lenses) sharpen what you see; and the convex lens in your eye focuses it all. Once you know “concave/convex” and “converge/diverge,” everyday optics makes sense.

Quick Reference

Optical element Effect on parallel light Image it forms
Concave mirror Converges Enlarged/erect (near) → inverted/smaller (far)
Convex mirror Diverges Always erect & diminished
Convex lens Converges Like a concave mirror (near erect/enlarged, far inverted)
Concave lens Diverges Always erect & diminished
Plane mirror Stays parallel Erect, same size
The two laws of reflection
1. Angle of incidence = angle of reflection (i = r).
2. Incident ray, normal, and reflected ray lie in one plane.

Memory Tricks & One-Page Revision

Revision Card

  • Concave caves in; convex bulges out. Same for mirrors and lenses.
  • Converging pair: concave mirror + convex lens (bring light together; can burn paper; can magnify or invert).
  • Diverging pair: convex mirror + concave lens (spread light; image always erect & diminished, wide view).
  • i = r, measured from the normal (the 90° line), not the mirror.
  • Light along the normal → i = r = 0°.
  • Convex mirror → “objects closer than they appear” (wide view, small image).
  • You look in a mirror but through a lens.

Spot the Mistake

Find the Flaw

  1. “A convex mirror can form an enlarged image.” — False: a convex mirror’s image is always erect and diminished.
  2. “The angle of incidence is measured from the mirror’s surface.” — False: it’s measured from the normal (the perpendicular).
  3. “A concave lens can be used as a magnifying glass.” — False: a convex lens magnifies; a concave lens always makes things look smaller.

Test Your Reflexes

A quick-fire drill: an optical element appears. Does it Converge light or Diverge it? Build a streak!

Converges or diverges?
Does it bring light together or spread it out?
Score 0 · Streak 0

Exam Corner: CBSE-Style Practice

Mixed Practice (objective, short, long, HOTS)

Objective type (1 mark each)

  1. The image in a convex mirror is always erect and ______.
  2. The angle of incidence is measured between the incident ray and the ______.
  3. A convex lens is thicker in the ______ than at the edges.

Short answer (2 marks each)

  1. State the two laws of reflection.
  2. Give one use each of a concave mirror and a convex mirror.

Long answer (3 marks each)

  1. Describe how the image in a concave mirror changes as an object is moved from near to far.
  2. Explain why a convex mirror is used as a vehicle’s side-view mirror, and what the warning “objects are closer than they appear” means.

HOTS (Higher Order Thinking)

  1. A ray hits a mirror making 40° with the mirror surface. What is the angle of incidence, and the angle of reflection? (Careful — angles are from the normal!)
  2. A magnifying glass shows enlarged text when held close, but as you lift it far away the text turns upside down. Which lens is it, and why does this happen?

Assertion–Reason (Choose: (a) both true, R explains A; (b) both true, R doesn’t explain A; (c) A true, R false; (d) A false, R true.)

  1. Assertion (A): Convex mirrors are preferred for observing traffic behind us.   Reason (R): A convex mirror gives a much wider field of view than a plane mirror.
  1. diminished (smaller).
  2. normal (the perpendicular at the point of incidence).
  3. middle.
    1. The angle of incidence equals the angle of reflection (i = r). (ii) The incident ray, the normal, and the reflected ray lie in the same plane.
  5. Concave: headlight/torch reflector, dentist’s mirror, telescope (any one). Convex: side-view mirror, road-safety/surveillance mirror (any one).
  6. Object near → erect and enlarged; as it moves away, the image becomes inverted and shrinks (first enlarged, then progressively smaller).
  7. A convex mirror always gives a small, erect image and a wide view of the road behind, so the driver sees more traffic. Because the image is diminished, vehicles look farther away than they really are — hence the warning that they are actually closer than they appear.
  8. A ray at 40° from the mirror surface makes 90° − 40° = 50° with the normal, so the angle of incidence = 50°, and the angle of reflection = 50° (since i = r).
  9. It is a convex (converging) lens. Held close, it gives an erect, enlarged image; moved far from the object, the image becomes inverted — the behaviour of a convex lens as object distance increases.
  10. (a) — Both true and R explains A: the wider field of view is exactly why convex mirrors are chosen for seeing traffic behind.

Glossary

Key Terms

  • Spherical mirror: a mirror curved like part of a sphere — concave (inward) or convex (outward).
  • Incident / reflected ray: the ray arriving at / leaving the mirror.
  • Normal: the line perpendicular to the surface at the point of incidence.
  • Angle of incidence / reflection: angles of those rays from the normal (i = r).
  • Converge / diverge: to bring light rays together / spread them apart.
  • Lens: transparent curved material; convex (thick middle, converging) or concave (thick edges, diverging).