Quick Revision Notes: Class 8 Science

Chapter: Light (प्रकाश)

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1. Introduction to Light

• 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).

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2. Reflection of Light (प्रकाश का परावर्तन)

• 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.

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3. Laws of Reflection (परावर्तन के नियम)

There are two main laws of reflection:

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

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4. Types of Reflection (परावर्तन के प्रकार)

• 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).

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5. Multiple Images (बहुप्रतिबिंब)

• 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).

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6. Sunlight - White or Colored? (सूर्य کا प्रकाश - श्वेत या रंगीन?)

• 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.

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7. Structure and Function of the Human Eye (मानव नेत्र की संरचना)

• 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.

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8. Care of the Eyes (नेत्रों की देखरेख)

• 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.

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9. Visually Impaired Persons (दृष्टिबाधित व्यक्ति)

• 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.