Chapter 13: Light
13.1 & 13.2 Laws of Reflection
What Makes Things Visible?
- We see an object only when light from it enters our eyes. This light may be emitted by the object or reflected by it.
- A polished or shiny surface (like a mirror) changes the direction of light falling on it.
Reflection Terminology
- Incident Ray: The light ray which strikes any reflecting surface.
- Reflected Ray: The light ray that comes back from the surface after reflection.
- Normal: An imaginary line drawn at an angle of 90° to the reflecting surface at the point of incidence.
- Angle of Incidence (∠i): The angle between the normal and the incident ray.
- Angle of Reflection (∠r): The angle between the normal and the reflected ray.
The Two Laws of Reflection
- First Law: The angle of incidence is always equal to the angle of reflection (∠i = ∠r).
- Second Law: The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.
- Lateral Inversion: An effect where the left of an object appears on the right in its mirror image, and vice versa.
13.3 Regular and Diffused Reflection
Types of Reflection
- Regular Reflection: Reflection from a smooth surface (like a mirror). Parallel incident rays are reflected parallel to each other, forming clear images.
- Diffused (Irregular) Reflection: Reflection from a rough or uneven surface (like cardboard). Parallel incident rays are reflected in different directions due to surface irregularities, not because of the failure of reflection laws.
Luminous vs. Illuminated Objects
- Illuminated Objects: Objects that shine by reflecting light from other sources (e.g., the Moon).
- Luminous Objects: Objects that emit their own light (e.g., the Sun, candles, electric lamps).
13.4 & 13.5 Multiple Reflections & Images
Multiple Images
- Two plane mirrors placed at an angle to each other form multiple images.
- The number of images formed depends on the angle between the mirrors.
- When mirrors are placed parallel to each other, an infinite number of images are formed.
Optical Instruments
- Periscope: Uses two parallel plane mirrors inclined at 45° to enable viewing of objects out of direct line of sight (used in submarines, tanks, and bunkers).
- Kaleidoscope: An instrument containing three rectangular mirror strips joined to form a prism. It creates beautiful, non-repeating patterns through multiple reflections of colored glass pieces.
13.6 Sunlight — White or Coloured
Dispersion of Light
- Sunlight (white light) consists of seven constituent colors.
- Dispersion: The splitting of white light into its individual colors (e.g., when passing through a prism or water droplets to form a rainbow).
13.7 & 13.8 Structure and Care of the Human Eye
Anatomy of the Eye
- Cornea: The tough, transparent outer front part of the eye that protects its interior.
- Iris: The dark muscular structure behind the cornea that gives the eye its distinctive color and controls the size of the pupil.
- Pupil: The small opening in the iris through which light enters.
- Lens: A crystalline structure behind the pupil that focuses light onto the back of the eye.
- Retina: The light-sensitive inner layer containing nerve cells that transmit visual signals to the brain.
- Optic Nerve: Transmits visual sensations from the retina to the brain.
Physiology of Vision
- The retina contains two types of photoreceptor cells:
- Cones: Sensitive to bright light; detect color.
- Rods: Sensitive to dim light.
- Blind Spot: The junction of the optic nerve and the retina where no sensory cells exist, making vision impossible at that spot.
- Persistence of Vision: The impression of an image persists on the retina for about 1/16th of a second. If static images are flashed faster than 16 frames per second, they appear as a continuous moving picture.
Care of the Eyes & Defects
- The most comfortable reading distance for a normal eye is about 25 cm.
- Cataract: A condition in old age where the eye lens becomes cloudy, causing foggy vision. It is treated by surgically replacing the opaque lens with an artificial one.
- Vitamin A Deficiency: Can lead to night blindness. Rich sources include carrots, broccoli, spinach, milk, eggs, and papaya.
13.9 & 13.10 Visually Impaired & Braille System
The Braille System
- Developed by Louis Braille in 1821 for visually challenged individuals.
- Consists of 63 dot patterns or characters arranged in cells of two vertical rows of three dots each.
- Characters represent letters, combinations of letters, common words, or grammatical signs. The dots are slightly raised to allow recognition by touch.