📌 Nature and Propagation of Light
- Light is a form of energy that makes objects visible to us.
- Luminous objects produce their own light (e.g. Sun, candle flame, electric bulb).
- Non-luminous objects do not produce light but can reflect it (e.g. Moon, book, wall).
- Light travels in straight lines — this is called the rectilinear propagation of light.
- Evidence: formation of shadows, eclipses (solar and lunar), pinhole camera.
- Light travels at approximately 3 × 10⁸ m/s in vacuum/air.
Pinhole Camera: Based on rectilinear propagation, a pinhole camera forms an inverted, real image of an object. The smaller the hole, the sharper the image.
🔄 Reflection of Light
When a ray of light falls on a smooth, shiny surface and bounces back, it is called reflection.
Laws of Reflection
- First Law: The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
- Second Law: The angle of incidence (∠i) is equal to the angle of reflection (∠r). i.e. ∠i = ∠r
Types of Reflection
- Regular (Specular) Reflection: Occurs on smooth, polished surfaces (mirrors, still water). Gives a clear image.
- Irregular (Diffuse) Reflection: Occurs on rough surfaces (paper, wall, uneven surfaces). Light scatters in all directions — no clear image formed.
🪞 Plane Mirrors
- A plane mirror is a flat, smooth, silvered glass surface.
- The image formed is virtual (cannot be projected on a screen), erect, and laterally inverted.
- The image is the same size as the object.
- The image is formed as far behind the mirror as the object is in front of it.
Lateral Inversion: In a plane mirror, the left side of an object appears as the right side in the image, and vice versa. This is why text appears reversed in a mirror.
🔵 Spherical Mirrors
Spherical mirrors are curved mirrors. They are parts of a hollow sphere with a reflecting surface.
| Property | Concave Mirror | Convex Mirror |
|---|---|---|
| Reflecting surface | Inner curved surface (cave-like) | Outer curved surface |
| Nature of image (distant object) | Real, inverted, small | Virtual, erect, small |
| Converging / Diverging | Converging mirror | Diverging mirror |
| Uses | Shaving/make-up mirrors, torches, headlights, dentist's mirror | Rear-view mirrors in vehicles, security mirrors in shops |
Important Terms
- Centre of Curvature (C): Centre of the sphere of which the mirror is a part.
- Radius of Curvature (R): Radius of that sphere.
- Principal Axis: The line passing through the centre of curvature and the pole.
- Principal Focus (F): Point where parallel rays converge (concave) or appear to diverge from (convex) after reflection.
- Focal Length (f): Distance between the pole and the focus. f = R/2
💡 Applications of Multiple Reflections
- Kaleidoscope: Uses two or three plane mirrors inclined at 60° to create beautiful, symmetrical patterns through multiple reflections.
- Periscope: Uses two plane mirrors inclined at 45° to see over obstacles or around corners. Used in submarines.
- Barber's mirrors: Two parallel plane mirrors facing each other create multiple images of an object placed between them.
🏷️ Key Terms
Luminous
Non-luminous
Rectilinear propagation
Shadow
Pinhole camera
Reflection
Angle of incidence
Angle of reflection
Normal
Lateral inversion
Plane mirror
Concave mirror
Convex mirror
Focal length
Periscope
Kaleidoscope