Light – Reflection and Refraction – Class 10 Science Chapter 9 Notes & NCERT PDF

Introduction

We see an object when light reflected from it reaches our eyes. A small source of light casts a sharp shadow of an opaque object, which shows that light travels in a straight line — a ray of light. (The book notes that when an obstacle is very small, light bends around it — diffraction — and that modern quantum theory treats light as neither purely a wave nor a particle.) In this chapter we use the straight-line (ray) model to study reflection by spherical mirrors and refraction by glass slabs and lenses.

Key Concepts

1. Laws of Reflection

(i) The angle of incidence is equal to the angle of reflection.

(ii) The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

These laws hold for all reflecting surfaces, including curved ones. A plane mirror gives an image that is virtual, erect, the same size as the object, as far behind the mirror as the object is in front, and laterally inverted.

2. Spherical Mirrors — the vocabulary

For mirrors of small aperture, R = 2f — so F lies midway between P and C.

In Activity 9.2, a concave mirror focuses sunlight into a bright spot that can burn paper; that spot is a real image of the Sun, and its distance from the mirror gives the approximate focal length.

3. Rules for Ray Diagrams (mirrors)

Use any two of these:

4. Image Formation by a Concave Mirror (Table 9.1)

Uses of concave mirrors: torches, search-lights and vehicle headlights (powerful parallel beams); shaving mirrors; dentists' mirrors (large image of teeth); solar furnaces.

5. Image Formation by a Convex Mirror (Table 9.2)

Uses of convex mirrors: rear-view (wing) mirrors in vehicles — they always give an erect, diminished image and have a wider field of view. The book mentions a convex mirror in a wall of Agra Fort that shows the full image of the Taj Mahal.

6. New Cartesian Sign Convention

Consequences: for a real object u is always negative; f of a concave mirror is negative, f of a convex mirror is positive.

7. Mirror Formula and Magnification

Mirror formula: 1/v + 1/u = 1/f

Magnification: m = h′/h = −v/u

A negative m means the image is real and inverted; a positive m means virtual and erect.

8. Refraction of Light

Refraction is the change in direction of light when it passes obliquely from one transparent medium to another, because the speed of light changes. Everyday signs: a pencil in water looks bent, a pond looks shallower, letters under a glass slab look raised, a lemon in a glass of water looks bigger. In Activity 9.8 a coin that had just disappeared from view becomes visible again when water is poured into the bowl.

Glass slab (Activity 9.10): at the air→glass face the ray bends towards the normal; at the glass→air face it bends away. The emergent ray is parallel to the incident ray but laterally displaced, because the bending at the two parallel faces is equal and opposite. A ray falling normally passes straight through.

Laws of refraction

(i) The incident ray, the refracted ray and the normal at the point of incidence lie in the same plane.

(ii) Snell's law: sin i / sin r = constant, for light of a given colour and a given pair of media.

9. Refractive Index

10. Spherical Lenses

Ray rules for lenses: (i) a ray parallel to the axis passes through F₂ after refraction (convex) or appears to diverge from F₁ (concave); (ii) a ray through F₁ (convex), or heading towards F (concave), emerges parallel to the axis; (iii) a ray through O goes straight on.

11. Image Formation by a Convex Lens (Table 9.4)

Concave lens (Table 9.5): always gives a virtual, erect, diminished image between F₁ and O (at F₁ when the object is at infinity).

12. Lens Formula, Magnification and Power

Lens formula: 1/v − 1/u = 1/f

Magnification: m = h′/h = v/u

Power: P = 1/f (f in metres); unit dioptre (D), 1 D = 1 m⁻¹

Convex lens → f and P positive; concave lens → f and P negative. Lenses in contact: P = P₁ + P₂ + P₃ + … (e.g. +2.0 D and +0.25 D give +2.25 D).

Worked Numericals

1. Rear-view mirror (Example 9.1). Convex mirror, R = +3.00 m, bus at u = −5.00 m.

f = R/2 = +1.50 m. 1/v = 1/f − 1/u = 1/1.50 + 1/5.00 = 6.50/7.50 → v ≈ +1.15 m (behind the mirror).

m = −v/u = −(1.15)/(−5.00) ≈ +0.23 → virtual, erect, diminished.

2. Concave mirror on a screen (Example 9.2). h = 4.0 cm, u = −25.0 cm, f = −15.0 cm.

1/v = 1/f − 1/u = −1/15 + 1/25 = −2/75 → v = −37.5 cm (screen 37.5 cm in front).

m = −v/u = −1.5, h′ = −6.0 cm → real, inverted, enlarged.

3. Exercise 15. h = 7.0 cm, u = −27 cm, concave f = −18 cm.

1/v = −1/18 + 1/27 = −1/54 → v = −54 cm; m = −(−54)/(−27) = −2; h′ = −14 cm (real, inverted, twice as large).

4. Convex lens (Example 9.4). h = 2.0 cm, f = +10 cm, u = −15 cm.

1/v = 1/f + 1/u = 1/10 − 1/15 = 1/30 → v = +30 cm; m = v/u = −2; h′ = −4.0 cm → real, inverted, enlarged.

5. Concave lens (Example 9.3). f = −15 cm, v = −10 cm.

1/u = 1/v − 1/f = −1/10 + 1/15 = −1/30 → u = −30 cm; m = v/u = +1/3 → virtual, erect, one-third size.

6. Speed and power. In glass of n = 1.50, v = c/n = 3 × 10⁸ / 1.50 = 2 × 10⁸ m s⁻¹. A lens of −2.0 D has f = −0.50 m (concave); +1.5 D has f ≈ +0.67 m (convex).

Formula Sheet

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Key Terms

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