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 → Waves and sound → Sound waves ↓


Reflection of sound and echo


Sound is a fascinating phenomenon that travels through air, water and even solids. When we speak, play music or make any sound, it travels around us in the form of waves. These waves are what enable us to hear. But sound doesn't only travel in a straight line; it can also bounce back from surfaces. This bouncing of sound waves is called reflection of sound. When this reflection of sound is so strong that it can be heard separately from the original sound, we call it an echo.

Understanding sound waves

Before getting into the reflection of sound, let's first understand what sound waves are. Sound waves are mechanical waves that are created by vibrating objects and travel from one place to another through a medium (such as air or water). These waves are longitudinal waves, which means that the vibrations occur in the same direction as the wave travels.

The speed of sound depends on the medium it travels through. For example, sound travels faster through water than it does through air because the molecules in water are closer together, making it easier to transfer vibrational energy from molecule to molecule.

        Speed of Sound (v) in Air ≈ 343 meters per second at 20°C

Reflection of sound

Just like light, sound can reflect from surfaces. When sound waves strike a surface, some of the wave's energy is absorbed while the rest of the energy is reflected back into the medium. The law of reflection for sound follows the same principle as for light:

        Angle of Incidence (i) = Angle of Reflection (r)

This means that if a sound wave hits a surface at a certain angle, it will reflect in the opposite direction to the normal (the imaginary line perpendicular to the surface) at the same angle. Here is a simple visual explanation:

Event reflection

In the visualization above, the solid line is the incident sound wave, and the dashed line is the reflected sound wave. The vertical line is the normal. The angle of incidence is equal to the angle of reflection.

Real life examples of sound reflection

Let's take a look at some everyday examples where sound reflection is noticeable:

  • Whispering galleries: In places such as domes or circular buildings, sound can travel along the walls due to reflection, allowing whispers to be heard clearly across the room.
  • Concert halls: The design of concert halls often uses reflection of sound to enhance the acoustics, ensuring that music and voices are projected clearly throughout the venue.
  • Yelling while standing next to a wall: If you yell while standing next to a large, flat wall, you may hear your voice coming back to you, though it won't be as clear as an echo.

What causes resonance?

An echo is a type of reflected sound wave that we hear differently from the original sound. For an echo to occur, two main conditions must be met:

  1. The surface must be large and hard enough to reflect the sound wave effectively.
  2. There should be sufficient distance between the source of sound and the reflecting surface to hear the echo clearly.

The science behind echoes

Echo occurs when a sound wave reflects from a surface and comes back to the listener after the original sound has stopped. The time delay allows our brain to perceive the original sound and the reflected sound as two separate entities. The minimum distance required to hear the echo is approximately:

        Minimum Distance = Speed of Sound (in air) * (Time for Echo / 2) Given Time for Echo is typically about 0.1 seconds for perceptible cases.

Thus, for sound traveling at 343 meters per second, the minimum distance is about 17.15 meters. This explains why we do not hear echoes in small rooms.

Applications of resonance

Resonance has a variety of practical uses in engineering, science, and nature:

  • Sonar: Ships and submarines use sonar systems to detect underwater objects or the sea floor by emitting sound waves and listening for the echo.
  • Animal echolocation: Bats and dolphins use echolocation to hunt and navigate. They emit sound waves and interpret the echo to understand their surroundings.
  • Architectural acoustics: Understanding resonance helps architects design spaces where sound reflection enhances or degrades acoustic quality as needed.

Exploring resonance with examples

Example 1: Resonance calculation

Suppose you stand 34.3 m away from a large rock and clap your hands. How long will it take you to hear the echo?

        Distance to Cliff = 34.3 meters Total Round Trip Distance = 34.3 * 2 = 68.6 meters Speed of Sound in Air = 343 m/s Time taken for Echo = Total Distance / Speed of Sound = 68.6 / 343 ≈ 0.2 seconds

So, you will hear your echo after about 0.2 seconds.

Example 2: Architectural design

When designing auditoriums, sound engineers test for echo and reverberation to ensure that speakers and performers can be heard clearly by the audience. Surfaces are arranged and materials are selected that will absorb sound energy in certain places and reflect it in other places to control reverberation and enhance sound clarity.

Conclusion

Reflection of sound and echo play a vital role in how we experience the world acoustically. Understanding these phenomena not only enriches our scientific knowledge but also provides practical benefits in fields as diverse as architecture, navigation and wildlife studies. With a simple clap of hands in front of a mountainous backdrop or standing in an acoustically reinforced hall, the effects of sound reflection are both fascinating and practical.


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Reflection of sound and echo

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