Grade 9
  1. Mechanics  1.1. Motion  1.1.1. Types of motion  1.1.2. Distance and displacement  1.1.3. Speed and Velocity  1.1.4. Acceleration  1.1.5. Graphical representation of motion  1.1.6. Equations of motion  1.1.7. Uniform and non-uniform motion  1.1.8. Free fall and acceleration due to gravity  1.1.9. Relative speed  1.1.10. Circular motion  1.2. Laws of force and motion  1.2.1. The concept of force  1.2.2. newton's first law of motion  1.2.3. Newton's second law of motion  1.2.4. Newton's third law of motion  1.2.5. Inertia and types of inertia  1.2.6. Motion  1.2.7. Conservation of momentum  1.2.8. Application of Newton's laws in daily life  1.2.9. Types of Forces (Contact and Non-Contact)  1.2.10. Friction and its effects  1.3. Gravitational force  1.3.1. Newton's law of universal gravitation  1.3.2. Importance of Gravitational Force  1.3.3. Acceleration due to gravity  1.3.4. Free fall and weightlessness  1.3.5. Mass and weight  1.3.6. Variation of g with height and depth  1.3.7. Kepler's laws of planetary motion  1.3.8. Satellites and their orbits  1.4. Work, Energy and Power  1.4.1. Concept of work  1.4.2. Positive and negative actions  1.4.3. Work done by constant and variable force  1.4.4. Energy and its forms  1.4.5. Kinetic energy and potential energy  1.4.6. Law of conservation of energy  1.4.8. Commercial unit of energy  1.4.9. Work–energy theorem  1.5. Simple machines  1.5.1. Types of simple machines  1.5.2. Mechanical advantage  1.5.3. Machine efficiency  1.5.4. Levers and their types  1.5.5. Pulleys and Inclined Planes  1.5.6. Gears and their uses
  2. Properties of matter  2.1. States of matter  2.1.1. Solids, liquids and gases  2.1.2. Properties of different states of matter  2.1.3. Change in state of matter  2.1.4. Plasma and Bose–Einstein condensate  2.2. Density and pressure  2.2.1. Concept of density and relative density  2.2.2. Pressure in solids, liquids and gases  2.2.3. Pascal's law and its applications  2.2.4. Atmospheric pressure and its variations  2.2.5. Surface tension and capillarity  2.3. Buoyancy and Archimedes' principle  2.3.1. Buoyant force  2.3.2. Archimedes principle  2.3.3. Applications of Archimedes' Principle  2.3.4. floating and sinking objects  2.3.5. Theory of Flotation and Archimedes' Principle
  3. Heat and Thermodynamics  3.1. Temperature and heat  3.1.1. Concept of heat and temperature  3.1.2. Temperature measurement  3.1.3. Temperature scales  3.2. Heat transfer  3.2.1. Conduction of heat  3.2.2. Convection of heat  3.2.3. heat radiation  3.2.4. Applications of heat transfer  3.2.5. Thermal conductivity  3.3. Thermal expansion  3.3.1. Expansion of solids, liquids and gases  3.3.2. Abnormal expansion of water  3.3.3. Applications of Thermal Expansion  3.4. Specific heat capacity and latent heat  3.4.1. Concept of specific heat capacity  3.4.2. Measurement and applications of specific heat  3.4.3. Latent heat of fusion and vaporization  3.4.4. Heat Engines and Efficiency
  4. Waves and sound  4.1. Waves and their types  4.1.1. Mechanical and electromagnetic waves  4.1.2. Longitudinal and Transverse Waves  4.1.3. Properties of Waves  4.1.4. Wavelength, frequency and speed of the wave  4.1.5. Superimposition of waves  4.2. Sound waves  4.2.1. Nature and transmission of sound  4.2.2. Speed of sound in different mediums  4.2.3. Reflection of sound and echo  4.2.4. Sonar and ultrasound  4.2.5. Resonance and pulsation  4.3. Sound characteristics  4.3.1. Intensity, loudness and quality of sound  4.3.2. Doppler effect in sound
  5. Lighting and Optics  5.1. Reflection of light  5.1.1. Laws of reflection  5.1.2. Reflection from a plane mirror  5.1.3. Reflection from a spherical mirror  5.1.4. Image formation by concave and convex mirrors  5.1.5. Applications of Reflection  5.2. Refraction of light  5.2.1. Laws of refraction  5.2.2. Refractive index  5.2.3. Refraction through a glass slab  5.2.4. Refraction in water and air  5.2.5. Lenses and their types  5.2.6. Image formation by convex and concave lenses  5.2.7. Total internal reflection and its applications  5.3. Dispersion and scattering of light  5.3.1. Spectrum of white light  5.3.2. Prisms and Dispersion  5.3.3. Tyndall effect and blue sky
  6. Electricity and Magnetism  6.1. Electric charge and static electricity  6.1.1. The concept of electric charge  6.1.2. Charging by friction, contact and induction  6.1.3. Electroscope and its working  6.2. Current Electricity  6.2.1. The concept of electric current  6.2.2. Ohm's Law and Resistance  6.2.3. Factors affecting resistance  6.2.4. Series and Parallel Circuits  6.2.5. Heating effect of electric current  6.2.6. Electric power and energy  6.3. Magnetism  6.3.1. Natural and artificial magnets  6.3.2. Properties of magnets  6.3.3. Earth's magnetism  6.3.4. Magnetic field and field lines  6.3.5. Electromagnets and their applications
  7. Modern Physics  7.1. structure of the atom  7.1.1. Atomic Structure and Subatomic Particles  7.1.2. Electrons, Protons, and Neutrons  7.1.3. Rutherford and Bohr model  7.2. Radioactivity  7.2.1. Types of radioactive emissions  7.2.2. Half-life and radioactive decay  7.2.3. Uses and Hazards of Radioactivity
  8. Space science and astronomy  8.1. The universe and its components  8.1.1. Stars and galaxies  8.1.2. Planets and Solar System  8.2. Satellites  8.2.1. Natural and artificial satellites  8.2.2. Use of satellites

Grade 9 → Lighting and Optics → Dispersion and scattering of light ↓


Prisms and Dispersion


When we see a rainbow in the sky, we see a beautiful spectrum of colours. This natural phenomenon occurs due to the dispersion of light, which is a fascinating concept in optics. To understand this phenomenon, we need to discuss the role of prisms, how they work, and the dispersion of light they cause. In simple terms, prisms can break down white light into its component colours, giving a clear view of the hidden mysteries of light.

What is a prism?

A prism is typically a transparent optical element with a flat, polished surface that refracts light. The most common form is a triangular prism, with a triangle as its base.

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A typical triangular prism

Prisms are made from a variety of materials, including glass and plastic. When light enters a prism, it bends, a behavior that results from a change in the speed of light as it moves from one medium to another, known as refraction.

Understanding the spread

Dispersion occurs because light is made up of different colors, each of which corresponds to a different wavelength. White light, like sunlight, is made up of a spectrum of colors ranging from red (the longest wavelength) to violet (the shortest wavelength).

When white light passes through a prism, each colour bends at a slightly different angle because of the difference in their wavelengths. This bending splits the light into its individual colours.

Process of refraction in a prism

As explained earlier, refraction occurs when light enters a new medium at an angle, causing its speed and direction to change. The extent to which light bends depends on two major factors:

  • The angle at which light strikes the surface of the medium (angle of incidence).
  • Variation in the refractive index of a medium for different wavelengths of light.

The refractive index is a dimensionless number that describes how quickly light travels through a material. Different materials and wavelengths have different refractive indices, which is why light is split into different colours.

Breaking down dispersion with a practical example

To understand how dispersion occurs in a simple way, let us imagine a ray of sunlight falling on a glass prism.

Step 1: Sunlight enters the prism.
Step 2: The light slows down and bends at the first surface.
Step 3: As the light passes through the prism, different colors bend at different angles.
Step 4: The colors emerge from the other side of the prism and spread out into a spectrum.

This sequence represents the dispersion of light and can be represented mathematically using Snell's law, which governs the refraction of light:

n1 * sin(θ1) = n2 * sin(θ2)

Where:

  • n1 and n2 represent the refractive indices of the two mediums.
  • θ1 is the angle of incidence.
  • θ2 is the angle of refraction.

Historical perspective

The phenomenon of dispersion was systematically studied by Sir Isaac Newton in the late 17th century. Using prisms, Newton demonstrated that white light is composed of different colors. He conducted an experiment in which he shone a ray of sunlight onto a prism, resulting in the dispersion of colors. Then, using another prism, he combined these colors back into white light, proving that color is a property of light itself.

Seeing the mess in everyday life

Dispersion is not just a subject of scientific laboratories or textbooks. We see this beautiful play of colours in many forms in our daily lives.

  • Rainbows: These are formed when sunlight is scattered by water droplets in the atmosphere, creating a spectrum of light.
  • CDs and DVDs: These discs create a spectrum similar to a prism when light reflects off of grooves on their surface.
  • Soap bubbles: Thin layers scatter light, making them appear coloured.

Creating a simple dispersion experiment at home

Here's how you can observe light dispersion at home with a simple setup:

  1. Find a prism or take a glass filled with water that is placed at an angle.
  2. Direct a beam of sunlight or flashlight light through the prism.
  3. Place a sheet of white paper on the other side to see the spectrum of colors.

Mathematics of dispersion

While the basic explanation is simple, dispersion relies on more complex mathematics involving theories of refractive index and wave behavior. When studying advanced optics, one delves deeper into equations that explain the change of refractive index at different wavelengths, sometimes described by Cauchy's equation:

n(λ) = A + (B / λ²) + (C / λ⁴)

Where:

  • λ is the wavelength of the light.
  • A, B and C are substance-specific constants.

Importance and applications of dispersion

Understanding dispersion has important implications in a variety of fields. Here are some examples:

  • Optical instruments: Telescopes and microscopes use lenses made of materials with different refractive indexes to minimise chromatic aberration (which is colour distortion due to dispersion).
  • Communications technologies: Optical fibers use the principles of dispersion to transmit data efficiently over long distances without signal loss.

Challenges in understanding dispersal

Despite its application, understanding dispersion presents challenges:

  • Understanding angles: Understanding how different angles affect spreads involves geometry and trigonometry.
  • Complex calculations: Advanced studies require a deeper understanding of wave behavior, which can be mathematically intensive.

Summary

Dispersion and the role of prisms provide a glimpse into the subtleties of light. Prisms don't just serve as tools in the laboratory; they open up a world of natural beauty and practical innovation, demonstrating the intricate dance of light through air and glass. By understanding these principles, we can better understand the colorful world around us, from rainbows to advanced optics.


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Prisms and Dispersion

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