Summary & Quick Quiz പ്രകാശത്തിന്റെ അപവർത്തനം
The whole chapter on one page: the eight key boxes of the textbook, every formula and fact you need, the mistakes to avoid, flashcards, a scored quiz and your study checklist.
1. The chapter in one picture
Read it clockwise from the top left: light has different speeds in different media (optical density, refractive index n = c/v); at an interface it changes direction – refraction – following four rules; refraction explains the raised coin, the broken straw and the twinkling stars; and from a denser to a rarer medium beyond the critical angle the light is totally reflected – fibres, reflectors, periscopes, diamonds and mirages.
2. The key boxes of the textbook
Light travels in a straight line within one medium. When a ray passes obliquely from one medium into another, its direction changes at the interface. A ray falling perpendicularly does not change direction. – page 5
The ability of a medium to affect the speed of light through it. Light is slower in an optically denser medium and faster in a rarer one. Optical density is not mass density. Order: air < water < glass < diamond. – page 5
When light passes obliquely from one medium into another of different optical density, its direction changes at the interface. This is refraction. – page 6
The ratio of the speed of light in vacuum to its speed in the medium. No unit. – page 6
(i) The incident ray, the refracted ray and the normal lie in the same plane. (ii) Rarer → denser obliquely: bends towards the normal. (iii) Denser → rarer: bends away from the normal. (iv) A ray falling normally is not refracted. – page 7
From a denser to a rarer medium, the angle of incidence for which the angle of refraction is 90° is the critical angle (glass–air 42°). At a greater angle of incidence the ray is not refracted but reflected completely back – total internal reflection. – page 10
When a critical angle is given for one medium alone, the second medium is air (or vacuum): “critical angle of water 48.6°” means water → air. – page 10
Charles K. Kao won the 2009 Nobel Prize in Physics for achievements in the transmission of light through optical fibres (ഒപ്റ്റിക്കൽ ഫൈബർ). – page 11
3. Formulas & facts
📐 Formulas & facts card
n = c/v ⇔ v = c/n · c = 3 × 108 m/s · n has no unit
| Medium | Speed of light (Table 1.1) | Refractive index | Critical angle (with air) |
|---|---|---|---|
| Air വായു | 3 × 108 m/s | 1 (1.0003) | – |
| Water ജലം | 2.25 × 108 m/s | 4/3 ≈ 1.33 | 48.6° |
| Glass ഗ്ലാസ് | 2 × 108 m/s | 3/2 = 1.5 (crown glass 1.52) | 42° |
| Diamond | 1.25 × 108 m/s | 2.4 (2.42) | 24.4° |
| Rule / fact | Remember |
|---|---|
| Rarer → denser, obliquely | bends towards the normal: r < i |
| Denser → rarer, obliquely | bends away from the normal: r > i |
| Along the normal (i = 0°) | no change of direction, r = 0° |
| Same plane | incident ray, refracted ray and normal lie in one plane |
| Same refractive index | no bending (glycerine 1.47 → sunflower oil 1.47) |
| Higher n | slower light, optically denser, smaller critical angle |
| Total internal reflection – conditions | (1) from a denser to a rarer medium; (2) angle of incidence greater than the critical angle |
| Glass slab | refraction at both faces; the emergent ray is parallel to the incident ray |
| Uses of total internal reflection | optical fibres (internet, endoscopy), reflectors, prism periscopes and binoculars, sparkle of diamonds, mirage, aquarium image (page 15) |
- Measuring the angle of incidence or refraction from the surface instead of from the normal. A ray at 30° to the surface has i = 60°.
- Thinking optical density = mass density. Kerosene is lighter than water (it floats) but optically denser (n 1.44 > 1.33).
- Saying light bends at normal incidence. A ray along the normal goes straight through.
- Mixing up towards and away. Rarer → denser = towards the normal; denser → rarer = away.
- Thinking total internal reflection can happen from rarer to denser, or at angles smaller than (or equal to) the critical angle.
- Giving refractive index a unit. It is a ratio of two speeds – no unit.
- Quoting a critical angle as a property of one medium alone without saying “with air” – the critical angle belongs to a pair of media.
- Writing “the star moves” instead of “the apparent position of the star changes” when explaining twinkling.
4. Flashcards
Click a card to flip it.
5. Scored quiz
Answer all fourteen, then press Check my answers. Each wrong answer shows the page to revise. For number answers type only the number.
6. Your study checklist
Ticks come from “Mark this page as completed” at the bottom of each page.
Next: the model test paper ▶ – attempt it on paper with a timer first, then check with the solved paper.