Light – Reflection and Refraction

REVISION SUMMARY: Light – Reflection and Refraction (NCERT Class 10, Ch 9)

1. Chapter at a glance

• Light appears to travel in straight lines; a ray of light is the straight-line path indicated by the direction of propagation.
• Laws of reflection hold for all reflecting surfaces, including spherical mirrors.
• Spherical mirrors (concave/convex) form images whose nature, position and size depend on object position relative to P, F and C.
• Refraction occurs when light travels obliquely from one transparent medium to another; it bends towards or away from the normal depending on the optical density of the media.
• For a rectangular glass slab, emergent ray is parallel to incident ray but laterally displaced.
• Spherical lenses (convex/concave) form images by refraction; convex lenses converge, concave lenses diverge parallel rays.
• New Cartesian Sign Convention is used for mirrors and lenses; mirror and lens formulae relate u, v and f.
• Power of a lens (P = 1/f) measures its ability to converge or diverge light; SI unit is dioptre (D) when f is in metres.

2. Definitions and laws (exact NCERT wording)

• Laws of reflection: (i) The angle of incidence is equal to the angle of reflection, and (ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
• Pole (P): The centre of the reflecting surface of a spherical mirror.
• Centre of curvature (C): The centre of the sphere of which the reflecting surface of a spherical mirror forms a part.
• Radius of curvature (R): The radius of the sphere of which the reflecting surface of a spherical mirror forms a part.
• Principal focus (F): Point on the principal axis where rays parallel to the principal axis meet (concave) or appear to diverge from (convex) after reflection.
• Focal length (f): The distance between the pole and the principal focus of a spherical mirror.
• Aperture: The diameter of the reflecting surface of the spherical mirror.
• For spherical mirrors of small apertures: R = 2f.
• Laws of refraction: (i) The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lie in the same plane. (ii) The ratio of sine of angle of incidence to the sine of angle of refraction is a constant, for the light of a given colour and for the given pair of media (Snell’s law): sin i / sin r = constant.
• Refractive index (n₂₁): n₂₁ = Speed of light in medium 1 / Speed of light in medium 2.
• Absolute refractive index of a medium (nₘ): nₘ = Speed of light in air / Speed of light in the medium = c / v.
• Optical centre (O) of a lens: Central point of the lens; a ray through it passes undeviated.
• Principal focus of a lens (F₁, F₂): Point on the principal axis where parallel rays converge (convex) or appear to diverge from (concave) after refraction.
• Focal length of a lens (f): Distance of the principal focus from the optical centre.
• Power of a lens: P = 1/f (SI unit: dioptre, D, when f is in metres). Power of convex lens is positive; concave lens is negative.

3. Important diagrams and activities

• Fig. 9.1: Schematic representation of concave and convex mirrors (shaded non-reflecting side).
• Fig. 9.2: Principal focus of concave and convex mirrors (parallel rays after reflection).
• Fig. 9.3–9.6: Four standard rays used for image location (parallel to axis, through F, through C, incident at P).
• Fig. 9.7: Ray diagrams for image formation by concave mirror at six object positions.
• Fig. 9.8: Image formation by convex mirror (object at infinity and finite distance).
• Fig. 9.10: Refraction through rectangular glass slab (incident, refracted, emergent rays; normal at both surfaces).
• Fig. 9.12: Converging action of convex lens and diverging action of concave lens.
• Fig. 9.13–9.15: Three standard rays for lenses.
• Fig. 9.16 & 9.17: Image formation by convex and concave lenses at various object positions.
• Activities 9.1–9.6: Image characteristics on spoon surfaces; focal length of concave mirror using sunlight; image positions with candle and screen for concave mirror; convex-mirror pencil images; full-length image in plane/concave/convex mirrors.
• Activities 9.7–9.11: Coin in water (refraction); coin in bowl with water; line under glass slab; pins through glass slab; focal length of convex lens using sunlight.
• Activities 9.12–9.13: Image positions with convex and concave lenses using lines at f intervals.

4. Common misconceptions and exam pitfalls

• Sign errors: forgetting New Cartesian convention (object distance always negative, focal length of concave mirror/convex lens negative).
• Assuming R = 2f for all apertures (valid only for small apertures).
• Confusing real vs virtual images or erect vs inverted when object is between P and F.
• Treating refractive index as mass density; forgetting that emergent ray in slab is parallel to incident ray.
• Using mirror formula for lenses or vice versa; omitting negative sign in magnification when image is real/inverted.
• Writing power without unit (D) or with f in cm instead of metres.

5. Formula sheet

All formulae must be used with the New Cartesian Sign Convention.