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 ↓


Tyndall effect and blue sky


Understanding the Tyndall effect and the blueness of the sky are interesting topics in optics. Both phenomena occur due to the dispersion and scattering of light. Let us understand each of these concepts in detail.

Tyndall effect

The Tyndall effect is a phenomenon observed when a beam of light is scattered by particles in a colloid or fine suspension. The effect is named after the 19th-century scientist John Tyndall, who extensively studied the scattering of light in gases.

Example 1: A beam of light in a dark room

Imagine you are in a dark room and someone turns on a flashlight. You see a beam of light moving across the room as it illuminates dust particles suspended in the air. This is an example of the Tyndall effect.

Dust Particles

Scientific explanation

In scientific terms, the Tyndall effect occurs when the diameter of the particles in a colloid or suspension is the same as or larger than the wavelength of the incident light. When this happens, the light is scattered, and we can see the path of the light beam.

diameter of particles ≥ wavelength of light

Applications of Tyndall effect

One practical application of the Tyndall effect is to determine whether a mixture is a true solution or a colloid. In a true solution, the particles are too small to scatter light significantly, so they will not exhibit the Tyndall effect.

Why is the sky blue?

The blue color of the sky is a classic example of light scattering, specifically Rayleigh scattering. Here is a simple explanation of why the sky appears blue during the day.

Rayleigh scattering

When sunlight enters the Earth's atmosphere, it collides with molecules and tiny particles in the air. Sunlight, or white light, is made up of many colors, each of which has a different wavelength. Blue light has a shorter wavelength than red light. According to Rayleigh scattering principles, shorter wavelengths are scattered more than longer wavelengths.

Intensity of scattered light ∝ 1 / (wavelength^4)

Therefore, blue light is scattered in all directions more effectively than other colors. This scattered blue light is what we see when we look up at the sky.

Visual example of Rayleigh scattering

Suppose sunlight is represented by a beam of white light entering a box symbolizing the atmosphere. Inside the box, molecules are scattering the blue component of the light more than the others, which helps us understand why we see a blue sky.

sunlight enters the atmosphere scattered blue light

Why no violet light?

Since violet light has a shorter wavelength than blue light, you may wonder why the sky doesn't appear purple. There are a few reasons for this:

  • The Sun emits less violet light than blue light.
  • The human eye is less sensitive to violet light.
  • The upper atmosphere absorbs some of the violet light.

Colors of the sky at different times of the day

When the sun is low in the sky, such as at sunrise or sunset, more of the sunlight passes through the Earth's atmosphere. Longer wavelengths such as red, orange and yellow dominate because much of the shorter wavelength blue light is scattered out of our line of sight. This is why we can see beautiful red and orange colors during these times.

sunset with red color

Conclusion

In short, the Tyndall effect and the color of the sky are amazing natural phenomena explained by the physics of light. The interaction between light and particles produces amazing visual effects that capture our imagination.


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Tyndall effect and blue sky

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