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 ↓


Spectrum of white light


Introduction

The study of light is a fascinating subject in the field of physics. Of the many phenomena associated with light, two of the most interesting are dispersion and scattering. These phenomena help us understand the spectrum of white light, which includes the range of colors visible to the human eye. In this lesson, we will explore how the spectrum of white light is formed, and what role dispersion and scattering play in this process.

What is white light?

White light is usually thought of as colorless or simply "white," but it is actually made up of many colors mixed together. When we look at sunlight or the light from a white LED bulb, we are actually seeing this mixture of colors. White light is made up of visible colors with different wavelengths, ranging from red to violet.

Understanding the spectrum of white light

The spectrum of white light is often represented through the dispersion of light. Dispersion is the separation of light into its component colors by refraction. This can be seen when light passes through a prism. The prism bends the light at different angles depending on the wavelength, resulting in a spectrum of colors.

Refraction Index of Materials:
  • Red: 1.520
  • Orange: 1.526
  • Yellow: 1.530
  • Green: 1.540
  • Blue: 1.550
  • Indigo: 1.560
  • Violet: 1.570

Visual example: A simple prism experiment

Consider a glass prism with a beam of white light entering through one of its faces:

White light is refracted at each surface of the prism. Each color in the light spectrum bends at a different angle because of the different refractive index. This refraction inside the prism spreads the light into a spectrum of colors.

Role of refraction and dispersion

Light travels at different speeds through different materials. This difference in speed occurs because materials have different optical densities. The refractive index of a medium determines how much it will bend or refract light as it passes through it. Different wavelengths of light, which we see as different colors, are each refracted by different amounts. This is what causes dispersion.

Snell's Law: n1 * sin(θ1) = n2 * sin(θ2)

where n1 and n2 are the refractive indices of the two media, and θ1 and θ2 are the angles of incidence and refraction, respectively.

Visible spectrum

The visible spectrum is a narrow band of the electromagnetic spectrum that can be seen by the human eye. The main colors seen range from red to violet:

  • Red: longest wavelength (about 700 nm)
  • Orange: (around 620 nm)
  • Yellow: (about 580 nm)
  • Green: (about 530 nm)
  • Blue: (about 470 nm)
  • Indigo: (about 425 nm)
  • Violet: shortest wavelength (about 400 nm)

Scattering of light

Scattering is another important phenomenon related to the spectrum of white light. It occurs when light is redirected while passing through a medium. It is particularly noticeable in the atmosphere and is caused by particles and gases scattering the light in different directions.

Visual example: Rayleigh scattering

incoming white light scattered light

Rayleigh scattering explains why the sky appears blue. Molecules in the atmosphere scatter shorter (blue) wavelengths of light more than longer (red) wavelengths. Because of this scattering effect, we see a blue sky instead of white during the day.

Everyday examples

The spectrum of white light is not just a theoretical concept, but can also be observed in everyday life.

Rainbow

Rainbows form when sunlight refracts and reflects inside raindrops, causing dispersion. Raindrops act like tiny prisms. As sunlight enters the raindrop, it refracts, causing the light to disperse into its component colors. Some of the light reflects off the back of the drop and refracts again on its way out, creating a beautiful spectrum arc in the sky.

Conditions for a Rainbow: 
1. Sunlight 
2. Water droplets in the air 
3. Observer between sunlight and the droplets

Soap bubbles

The glowing colors on a soap bubble are caused by the interference of light waves reflecting off the thin soap film. Some light reflects off the outer surface of the film, while other light enters the film and reflects off the inner surface. The differences in the paths taken by these light waves cause them to interfere with each other, causing some colors to be enhanced and others to be cancelled out.

Sunset and sunrise

The color of the sky changes at sunset and sunrise due to the longer path of light through the atmosphere. As the Sun is lower in the sky, red and orange wavelengths dominate as the shorter blue wavelengths are scattered out of the line of sight. This phenomenon is amplified by atmospheric particles, resulting in bright colors at the horizon.

Dispersion vs. scattering: Key differences

Although both involve the propagation of light, dispersion and scattering have different mechanisms and effects:

  • Dispersion is specifically caused by different wavelengths bending at different angles, primarily through refraction.
  • Scattering occurs when light is deflected by particles or molecules without changing the wavelength, which can be clearly seen in Rayleigh scattering.

Conclusion

The spectrum of white light reveals a world of vibrant colors and fascinating principles of physics. Understanding how light propagates through prisms and scatters through the atmosphere enriches our understanding of everyday visual phenomena. As we explore refraction, dispersion, and scattering, we gain insight into both the physics of light and the breathtaking beauty it creates in the natural world.


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Spectrum of white light

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