REVISION SUMMARY: Light: Mirrors and Lenses (Chapter 10)
1. Chapter at a Glance
- Spherical mirrors are formed when the reflecting surface of a mirror is a part of a hollow sphere; they are of two types — concave (curved inwards) and convex (curved outwards).
- When an object is placed close to a concave mirror, the image is erect and enlarged; as the object is moved farther, the image becomes inverted, initially enlarged and then diminished.
- A convex mirror always forms an erect and diminished image; the size of the image decreases slightly as the object moves away.
- The two laws of reflection are: (i) angle of incidence equals angle of reflection, and (ii) the incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.
- Laws of reflection hold for plane, concave and convex mirrors.
- A concave mirror converges parallel beams of light while a convex mirror diverges them.
- A convex lens (thicker at middle) forms images that can be erect/enlarged or inverted/diminished depending on object distance; a concave lens (thicker at edges) always forms an erect and diminished image.
- A convex lens converges parallel beams of light (also called converging lens) while a concave lens diverges them (diverging lens).
2. Definitions and Laws
- “Mirrors, whose reflecting surfaces are spherical are called spherical mirrors.”
- “A spherical mirror, which has a reflecting surface that curves inwards, is called a concave mirror.”
- “A spherical mirror which has a reflecting surface that curves outwards is called a convex mirror.”
- “The ray of light that falls on the mirror is called the incident ray. The ray of light that comes back from the mirror is called the reflected ray.”
- “The angle between the normal and the incident ray is called the angle of incidence (i). The angle between the normal and the reflected ray is known as the angle of reflection (r).”
- “The incident ray, the normal to the mirror at the point of incidence, and the reflected ray, all lie in the same plane.”
- “A lens is a piece of transparent material, usually made of glass or plastic, which has curved surfaces.”
- “A lens which is thicker at the middle as compared to the edges is called a convex lens.”
- “A lens which is thicker at the edges as compared to the middle is called a concave lens.”
- “A convex lens is also called a converging lens while a concave lens is called a diverging lens.”
(No numerical formulas or SI units appear in the chapter.)
3. Important Diagrams and Activities
- Fig. 10.1 (shiny spoon inner/outer surfaces) — shows erect vs inverted and enlarged vs diminished images.
- Fig. 10.2 & 10.3 (concave & convex mirrors with representations) — illustrates inward/outward curvature and shaded non-reflecting surface.
- Activity 10.3 & Fig. 10.5 — object at small/large distance from concave/convex mirrors to observe image characteristics.
- Fig. 10.6 & 10.7 — uses of concave mirrors (torch reflector, dentist’s mirror) and convex mirrors (vehicle side-view, road safety, shop surveillance).
- Fig. 10.9 — construction of incident ray, reflected ray, normal, angle of incidence (i) and angle of reflection (r).
- Activity 10.6 & Fig. 10.11 — multiple parallel rays on plane/concave/convex mirrors showing parallel, converging and diverging behaviour.
- Fig. 10.12 & 10.19 — concave mirror/convex lens focusing sunlight to burn paper (convergence).
- Fig. 10.15 & 10.16 — convex and concave lenses with representations.
- Activity 10.9 & Fig. 10.17 — object viewed through convex/concave lens at different distances.
- Activity 10.10 & Fig. 10.18 — multiple parallel rays through glass plate, convex lens and concave lens.
4. Common Misconceptions and Exam Pitfalls
- Assuming a concave mirror always gives inverted image (it gives erect enlarged image when object is close).
- Thinking convex mirror can give inverted image (it is always erect and diminished).
- Confusing angle of incidence with angle between ray and mirror surface (it is always measured from normal).
- Believing laws of reflection apply only to plane mirrors (they apply to all mirrors).
- Mixing up convex/concave lenses with mirrors (lenses allow light to pass through; mirrors reflect).
- Forgetting that image through concave lens or convex mirror is always erect and diminished.
5. Formula Sheet
No numerical formulas are given in the chapter.