Class 8 — Chapter 13: Light

13.1 & 13.2 Laws of Reflection

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.

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.

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

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.

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

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.

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

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

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.

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.

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

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.

Science: Class 8 — Chapter 13: Light