Quick Revision Notes: Class 8 Science

Chapter: Light (प्रकाश)

Light (प्रकाश): It is a form of energy that enables us to see objects. It travels in a straight line.
Luminous Objects (दीpt वस्तुएँ): Objects that emit their own light (e.g., Sun, torch, firefly).
Non-luminous Objects (अदीpt वस्तुएँ): Objects that do not emit their own light but reflect light falling on them from luminous objects (e.g., Moon, tree, book).

Reflection: The bouncing back of light into the same medium when it strikes a polished, shining surface.
Incident Ray (आपतित किरण): The light ray that strikes any surface.
Reflected Ray (पравर्तिt किरण): The ray that bounces back from the surface.
Normal (अभिलंब): A line perpendicular (at 90 degrees) to the reflecting surface at the point of incidence.
Angle of Incidence (∠i): The angle between the incident ray and the normal.
Angle of Reflection (∠r): The angle between the reflected ray and the normal.

There are two main laws of reflection:

First Law: The angle of incidence is always equal to the angle of reflection. $$\angle i = \angle r$$
Second Law: The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.

Regular Reflection (नियमित परावर्तन):
- Occurs on smooth and polished surfaces (e.g., plane mirror).
- Parallel incident rays remain parallel after reflection, forming a clear image.
Irregular or Diffused Reflection (विसरत परावर्तन):
- Occurs on rough or uneven surfaces (e.g., cardboard, wall).
- Parallel incident rays are reflected in different directions. This helps us see objects from all directions. (Note: Diffused reflection is not due to the failure of the laws of reflection).

When two mirrors are inclined at an angle to each other, they form multiple images.
Formula for Number of Images ($n$): If two plane mirrors are placed at an angle $\theta$ (theta) to each other, the number of images formed is given by: $$n = \left(\frac{360^\circ}{\theta}\right) - 1$$ (Provided $\frac{360^\circ}{\theta}$ is an even number).
Kaleidoscope (कैलाइडोस्कोप): An instrument based on the principle of multiple reflections in inclined mirrors to create beautiful patterns.
Periscope (परिदर्शी): Uses two plane mirrors placed at an angle of $45^\circ$ inside a tube. It allows us to see objects that are not directly in our line of sight (used in submarines).

Dispersion of Light (प्रकाश का विक्षेपण): Sunlight is referred to as white light, which consists of seven colors (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red).
The splitting of white light into its constituent colors is called dispersion. Example: Rainbow.

Cornea (कॉर्निया): The outer transparent front part of the eye that allows light to enter.
Iris (परितारिका): The colored part of the eye that controls the amount of light entering by adjusting the size of the pupil.
Pupil (पुतली): The small opening in the iris through which light enters the eye. It shrinks in bright light and expands in dim light.
Lens (नेत्र लेंस): A convex lens behind the pupil that focuses light on the retina.
Retina (दृष्टिपटल): The inner back layer of the eye where images are formed. It contains two types of light-sensitive cells:
1. Cone Cells (शंकु कोशिकाएँ): Sensitive to bright light and color.
2. Rod Cells (शलाका कोशिकाएँ): Sensitive to dim light.
Optic Nerve (दृक तंत्रिका): Transports visual signals from the retina to the brain.
Blind Spot (अंध बिंदु): The point at the junction of the optic nerve and retina where no vision is possible because no sensory cells are present.
Persistence of Vision (दृष्टि निर्बंध): The image of an object persists on the retina for about $\frac{1}{16}$th of a second.

Do not read in too dim or too bright light.
Never look directly at the Sun or a powerful laser light.
Wash your eyes frequently with clean water.
Maintain a normal distance (at least 25 cm) while reading.
Eat a balanced diet rich in Vitamin A (e.g., carrots, broccoli, eggs, milk, papaya, mango) to prevent night blindness.

Braille System (ब्रेल लिपि): A tactile reading and writing system developed by Louis Braille for visually impaired persons.
It uses a grid of 6 raised dots representing letters, numbers, and musical notes. Characters can be read by passing fingers over the raised dots.