Tamil Nadu Board of Secondary EducationSSLC (English Medium) Class 9

# Echoes

#### Topics

• ##### Force
• Force
• Translational and Rotational Motions
• Moment (Turning Effect) of a Force Or Torque
• Couple
• Equilibrium of Bodies and Its Types
• Principle of Moments
• Centre of Gravity
• Uniform Circular Motion (UCM)
• Centripetal Force
• Centrifugal Forces
• ##### Machines
• Machines
• Simple Machines
• Technical Terms Related to a Machine
• Principle of Machine
• Relationship between efficiency (ղ), mechanical advantage (M.A.) and velocity ratio (VR)
• Lever
• Kinds of Levers
• Examples of Each Class of Levers as Found in the Human Body
• Pulley
• Single Fixed Pulley
• Single Movable Pulley
• Combination of Pulleys
• Machines (Numerical)
• ##### Refraction of Light at Plane Surfaces
• Refraction of Light
• Law of Refraction of Light
• Speed of Light
• Relationship Between Refractive Index and Speed of Light (µ = C/V)
• Principle of Reversibility of the Path of Light
• Experimental Verification of Law of Refraction and Determination of Refractive Index of Glass
• Refraction of Light Through a Rectangular Glass Slab
• Multiple Images in a Thick Plane Glass Plate Or Thick Mirror
• Prism
• Refraction of Light Through a Prism
• Real and Apparent Depth
• Apparent Bending of a Stick Under Water
• Some Consequences of Refraction of Light
• Transmission of Light from a Denser Medium (Glass Or Water) to a Rarer Medium (Air) at Different Angles of Incidence
• Critical Angle
• Relationship Between the Critical Angle and the Refractive Index (µ = 1/ Sin C)
• Total Internal Reflection
• Total Internal Reflection in a Prism
• Use of a Total Internal Reflecting Prism in Place of a Plane Mirror
• Consequences of Total Internal Refraction
• ##### Spectrum
• Deviation Produced by a Triangular Prism
• Colour in White Light with Their Wavelength and Frequency Range
• Dispersion of Light Through Prism and Formation of Spectrum
• Electromagnetic Spectrum
• Different Radiation of Electromagnetic Spectrum
• Gamma Rays
• X rays
• Visible Light
• Micro Waves
• Scattering of Light and Its Types
• Applications of Scattering of Light
• ##### Sound
• Sound
• Difference Between the Sound and Light Waves
• Reflection of Sound
• Echoes
• Determination of Speed of Sound by the Method of Echo
• Use of Echoes
• Natural Vibrations
• Damped Vibrations
• Forced Vibrations
• Resonance
• Demonstration of Resonance
• Some Examples of Resonance
• Properties of Sounds
• Loudness and Intensity
• Pitch (or shrillness) and frequency
• Audibility and Range
• Quality (Or Timbre) and Wave Form
• Noise Pollution
• Noise and Music
• Sound (Numerical)
• ##### Household Circuits
• Transmission of Power from the Power Generating Station to the Consumer
• Power Distribution to a House
• House Wiring (Ring System)
• Electric Fuse
• Miniature Circuit Breaker (MCB)
• Electric Switch
• Circuits with Dual Control Switches (Staircase Wire)
• Earthing (Grounding)
• Three-pin Plug and Socket
• Colour Coding of Live, Neutral, and Earth Wires
• High Tension Wires
• Precautions to Be Taken While Using Electricity
• ##### Calorimetry
• Heat and Its Unit
• Temperatures
• Factors Affecting the Quantity of Heat Absorbed to Increase the Temperature of a Body
• Difference Between Heat and Temperature
• Thermal Capacity (Heat Capacity)
• Specific Heat Capacity
• Relationship Between the Heat Capacity and Specfic Heat Capacity
• Specific Heat Capacity of Some Common Substances
• Calorimetry and Calorimeter
• Principle of Method of Mixtures (or Principle of Calorimetry)
• Natural Phenomena and Consequences of High Specific Heat Capacity of Water
• Some Examples of High and Low Heat Capacity
• Change of State of Matter
• Melting and Freezing
• Heating Curve of Ice During Melting
• Change in Volume on Melting
• Effect of Pressure on the Melting Point
• Effect of Impurities on the Melting Point
• Concept of Boiling (Vaporization)
• Heating Curve for Water
• Change in Volume on Boiling
• Effect of Pressure on the Boiling Point
• Effect of Impurities on the Boiling Point
• Latent Heat and Specific Latent Heat
• Specific Latent Heat of Fusion of Ice
• Explanation of Latent Heat of Melting on the Basis of Kinetic Model
• Natural Consequences of High Specific Latent Heat of Fusion of Ice
• Structure of the Atom and Nucleus
• Atomic Model
• Isotopes
• Isobars
• Isotones or Isoneutronic
• Properties of Alpha Particles
• Properties of Beta Particles
• Changes Within the Nucleus in Alpha, Beta and Gamma Emission
• Alpha Decay (Alpha Emission)
• Beta Decay (Beta Emission)
• Gamma Decay (Gamma Emission)
• Safety Precautions While Using Nuclear Energy
• Nuclear Energy
• Nuclear Fission
• Distinction Between the Radioactive Decay and Nuclear Fission
• Nuclear Fusion
• Distinction Between the Nuclear Fission and Nuclear Fusion
• Production (or generation) of an echo
• Conditions necessary for hearing echo

## Notes

### Echo:

• If we shout or clap near a suitable reflecting object such as a tall building or a mountain, we will hear the same sound again a little later. This sound which we hear is called an echo. The sensation of sound persists in our brain for about 0.1 s.
• To hear a distinct echo the time interval between the original sound and the reflected one must be  at least 0.1s. If we take the speed of sound to be 344 m/s at a given temperature, say at 22 ºC in air, the sound must go to the obstacle and reach back the ear of the listener on reflection after 0.1s.
• Hence, the total distance covered by the sound from the point of generation to the reflecting surface and back should be at least (344 m/s) × 0.1 s = 34.4 m.
• Thus, for hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be half of this distance, that is, 17.2 m. This distance will change with the temperature of air.
• Echoes may be heard more than once due to successive or multiple reflections. The rolling of thunder is due to the successive reflections of the sound from a number of reflecting surfaces, such as the clouds and the land.
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Reflection of Sound: Echo [00:15:56]
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