Breaf Notes of Chapter 16: Light

 

1. Introduction to Light

• Light is a form of energy that enables us to see the world around us.
• It travels in straight lines (rectilinear propagation of light).
• Light travels at a very high speed: 3 × 10⁸ m/s (300,000 km/s) in vacuum/air.
• Sources of light: → Natural: Sun, stars, fireflies (bioluminescence), lightning → Artificial: Bulb, tube light, candle, laser, LED

2. Types of Objects Based on Light

(i) Luminous objects: Emit their own light (e.g., Sun, bulb). (ii) Non-luminous objects: Do not emit light but can be seen when light falls on them (e.g., moon, table, book).

3. Reflection of Light

• When light falls on a surface and bounces back into the same medium, it is called reflection.
• Laws of Reflection (always hold true):
  1. Angle of incidence (i) = Angle of reflection (r) ∠i = ∠r
  2. The incident ray, reflected ray, and normal at the point of incidence all lie in the same plane.

4. Terms Related to Reflection

• Incident ray: Ray of light falling on the mirror.
• Reflected ray: Ray that bounces back.
• Normal: Perpendicular line drawn at the point of incidence.
• Point of incidence: Point where incident ray meets the mirror.
• Angle of incidence (∠i): Angle between incident ray and normal.
• Angle of reflection (∠r): Angle between reflected ray and normal.

5. Types of Reflection

1. Regular reflection → Occurs on smooth, polished surfaces (plane mirror). → Forms clear and sharp image.
2. Diffused/Irregular reflection → Occurs on rough surfaces (wall, paper). → Light scatters in all directions. → No clear image formed, but surface is visible. → Reason we can see most objects.

6. Plane Mirror – Characteristics of Image

• Image is virtual (cannot be obtained on screen).
• Image is erect.
• Image is laterally inverted (left appears right and vice versa).
• Image is of same size as object.
• Image distance = Object distance from mirror.
• Image is formed as far behind the mirror as the object is in front.

7. Multiple Images by Plane Mirrors

• When two mirrors are placed at an angle θ, number of images formed = (360°/θ) – 1 Example: → 90° → 3 images → 60° → 5 images → 120° → 2 images

8. Kaleidoscope

• Works on the principle of multiple reflections.
• Contains three plane mirrors inclined at 60° to each other.
• Produces beautiful symmetrical patterns.

9. Periscope

• Used in submarines and tanks to see above water/ground.
• Uses two plane mirrors placed parallel at 45° angle.
• Light reflects twice and allows viewing over obstacles.

10. Dispersion of Light

• Splitting of white light into its seven constituent colours (VIBGYOR) when it passes through a prism.
• Cause: Different colours travel at different speeds in a medium → bend by different amounts (refraction).
• Order of colours: Violet bends most, Red bends least.

11. Rainbow

• Natural phenomenon of dispersion.
• Formed when sunlight passes through tiny raindrops acting as prisms.
• Two types: Primary rainbow (brighter, red on top), Secondary rainbow (fainter, violet on top).

12. Human Eye – Structure and Functioning

• Natural optical instrument. Main parts and their functions:

1. Cornea: Transparent front part; refracts light.
2. Iris: Controls size of pupil (coloured part).
3. Pupil: Hole that allows light to enter.
4. Eye lens: Crystalline, convex lens; forms real, inverted image on retina.
5. Ciliary muscles: Change curvature of lens for accommodation.
6. Retina: Screen where image is formed; contains rods (dim light) and cones (colours & bright light).
7. Optic nerve: Carries image signals to brain.

• Persistence of vision: Image remains on retina for 1/16th second → helps in watching movies.

13. How We See Objects

• Light from object → enters eye → refracted by cornea & lens → real, inverted image on retina → brain interprets as erect.

14. Care of Eyes

• Wash eyes with cold water.
• Read at proper distance (25–30 cm) and in sufficient light.
• Do not look at the sun directly.
• Eat foods rich in Vitamin A (carrot, amla, papaya, green leafy vegetables).
• Visit eye specialist if irritation persists.

15. Defects of Vision and Corrections

1. Myopia (Near-sightedness)
   • Can see near objects clearly, distant objects blurred.
   • Cause: Eyeball elongated or lens too convex.
   • Image formed in front of retina.
   • Correction: Concave lens (diverging lens).
2. Hypermetropia (Far-sightedness)
   • Can see distant objects clearly, near objects blurred.
   • Cause: Eyeball shortened or lens too flat.
   • Image formed behind retina.
   • Correction: Convex lens (converging lens).
3. Presbyopia
   • Old age problem; cannot see near and distant objects clearly.
   • Due to weakening of ciliary muscles.
   • Correction: Bifocal lenses (upper part concave, lower part convex).
4. Cataract
   • Lens becomes cloudy/opaque.
   • Surgery: Replace natural lens with artificial lens.

16. Visually Impaired Persons – Braille System

• Developed by Louis Braille.
• Uses raised dots (6-dot cell) to represent letters/numbers.
• Blind people read by touching.
• Modern aids: Screen reading software, audiobooks, talking watches.

17. Important Points to Remember

• Light always travels in straight lines → proved by pinhole camera and shadows.
• Speed of light is highest in vacuum.
• We see the moon because it reflects sunlight.
• Sun appears reddish at sunrise/sunset due to scattering of light (blue light scatters more, red reaches us).
• Atmosphere scatters blue light → sky appears blue.

Key Diagrams to Practice

1. Ray diagram showing laws of reflection.
2. Image formation by plane mirror.
3. Multiple images by two inclined mirrors.
4. Dispersion through prism.
5. Structure of human eye.
6. Defects of vision (myopia, hypermetropia) with ray diagrams and correction.

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