Optics — Class 10 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 10 Science, Physics — Chapter 2. How light bends, lenses form images, and how vision defects are corrected.
1. About this chapter
This chapter studies the refraction (bending) of light, the refractive index, total internal reflection, image formation by lenses, and the human eye with its common defects of vision.
2. Refraction and refractive index
- Refraction: the change in direction of light as it passes from one transparent medium to another due to a change in speed.
- Laws of refraction (Snell's law):
- The incident ray, refracted ray and normal lie in the same plane.
- sin i / sin r = constant = n (refractive index).
- Absolute refractive index: n = c / v (speed of light in vacuum ÷ speed in medium). Denser medium → higher n → light bends towards the normal.
- Glass slab: the emergent ray is parallel to the incident ray but laterally displaced.
3. Total internal reflection (TIR)
When light travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle (C), it is completely reflected back.
- Condition: ray goes denser → rarer, and i > C.
- n = 1 / sin C.
- Applications: mirage, brilliance of diamond, optical fibres, and total-reflecting prisms.
4. Lenses
| Term | Formula | Notes |
|---|---|---|
| Lens formula | 1/v − 1/u = 1/f | sign convention applies |
| Magnification | m = h'/h = v/u | +ve = erect/virtual, −ve = inverted/real |
| Power of a lens | P = 1/f (in metres) | unit dioptre (D); convex +, concave − |
- Convex (converging) lens: thicker at the middle; convex lens can form real or virtual images.
- Concave (diverging) lens: always forms a virtual, erect, diminished image.
5. The eye and defects of vision
- The eye focuses light on the retina; the ciliary muscles change the lens focal length (accommodation).
- Myopia (short sight): distant objects blurred; image forms before the retina; corrected by a concave lens.
- Hypermetropia (long sight): near objects blurred; image forms behind the retina; corrected by a convex lens.
- Presbyopia: loss of accommodation with age; corrected by bifocal lenses.
6. Worked examples
Example 1. Light travels at 2×10⁸ m s⁻¹ in glass. Find its refractive index. (c = 3×10⁸) n = c/v = 3×10⁸ / 2×10⁸ = 1.5.
Example 2. A convex lens has focal length 20 cm. Find its power. P = 1/f = 1/0.20 = +5 D.
Example 3. The critical angle of a medium is 30°. Find its refractive index. n = 1/sin C = 1/sin 30° = 1/0.5 = 2.
7. Common mistakes
- Mistake: Forgetting the sign convention in the lens formula. Fix: Distances measured against the incident light are negative; use the convention consistently.
- Mistake: Using f in cm for power. Fix: Power needs f in metres (P = 1/f).
- Mistake: Swapping myopia and hypermetropia corrections. Fix: Myopia → concave lens; hypermetropia → convex lens.
8. Practice (book-back style)
- State Snell's law of refraction.
- Define refractive index and write n = c/v.
- What is total internal reflection? Give two applications.
- A lens has power −2 D. Find its focal length and type.
- Name and correct the two common defects of vision.
9. Answer key
- sin i / sin r = constant (n); incident, refracted rays and normal are coplanar.
- Ratio of speed of light in vacuum to that in the medium; n = c/v.
- Complete reflection of light at a denser–rarer boundary when i > C; optical fibre, mirage (also diamond brilliance).
- f = 1/P = 1/(−2) = −0.5 m = −50 cm; concave (diverging) lens.
- Myopia → concave lens; hypermetropia → convex lens.
10. Quick revision
- Physics Ch 2 · refraction, refractive index, TIR, lenses, eye.
- Snell: sin i/sin r = n; n = c/v; denser → bends towards normal.
- TIR: denser→rarer and i > C; n = 1/sin C.
- Lens: 1/v − 1/u = 1/f; m = v/u; P = 1/f (D).
- Myopia → concave; hypermetropia → convex.
