nature waves

8/10/2019 Nature Waves

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WAVES


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Describing wave motion

Term Description SI units

Amplitude, A The maximum displacement of the ropefrom the rest position

Metre (m)

Wavelength, The shortest distance between 2 successivecrests or troughs

Metre (m)

Frequency, f The number of complete waves producedper second

Hertz (Hz)

Period, T The time taken to produce one completewave

Second (s)

Wave speed, v The distance travelled by a wave in 1 second Second (s)


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Describing wave motion

trough

crest


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Types of Waves Transverse waves

The vibration of the particles in the medium is perpendicular to thedirection in which the wave travels

Eg. water waves, rope waves, all types of electromagnetic wavesincluding light waves, microwaves, X-rays, gamma rays

The highest point reached by a vibrating particle in a transverse waveis called crest or peak while the lowest point is called trough

Longitudinal waves The vibration of the particles in the medium is parallel to the direction

in which the wave travels Eg. sound waves The section in which the vibrating particles in a longitudinal wave are

closest together is called compression while the section in which thevibrating particles are furthest apart is called rarefaction


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Wavefronts Any line or surface over which all the vibrating particles

are in the same phase Particles in the same phase have the same speed and

are at equal distances from their source In transverse waves, wavefronts are normally lines joining

all the peaks at equal distance from their source The distance between successive wavefronts equals a

wavelength The direction of travel of a wave is always perpendicular

to its wavefronts as indicated by lines drawnperpendicular to the wavefronts.


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Wave Equation Velocity of wave, v = f Example: The speed of light in vacuum is 3 x 10 8 m/sCalculate the frequency of orange light, given that its

wavelength in vacuum is 6 x 107

m.

3 x 10 8 = f x 6 x 10 -7

f = (3 x 10 8)/(6 x 10 -7) = 5 x 1 014 Hz


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Reflection of waves Waves are reflected when an obstacle is placed in their

paths All reflected waves obey the law of reflection which states

The angle of reflection equals the angle of incidence The incident wave, the reflected wave, and the normal all lie on the

same plane


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Properties of reflected waves The reflected wave the same wavelength, frequency, and

speed as the incident wave The velocities of the reflected and incident waves are

different because they travel in different directions The angle of reflection equal to the angle of incidence


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Refraction of waves Waves are refracted when their speeds are changed The speed of a wave is changed when the wave moves

from a dense medium into a less dense medium or fromdeep water to shallower water

If the incident wave is travelling along the normal, it willcontinue to travel along the normal after entering water ofa different depth

In all other cases, refraction produces a change in wavedirection

On entering shallower water, the wave direction bendstowards the normal.

On entering deeper water, the wave direction bends awayfrom the normal


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Refraction of water waves


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Refraction of waves

Properties Shallower to deeperwater

Deeper to shallowerwater

Wavelength Increases Decreases

Frequency Unchanged UnchangedSpeed Increases Decreases

Velocity Increases Decreases

Direction of travel Bends away from normal Bends towards normal


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Sound Production of sound waves by vibrating sources:

sound is produced by vibrating sources (egtuning fork) placed in a medium (solid, liquid, gas)

Nature of sound waves It is a form of energy that can be transferred from one point to another It is an example of longitudinal waves consisting of compressions and

rarefactions Compressions are regions where air pressure is slightly higher than he

surrounding air pressure Rarefactions are regions where air pressure is slightly lower than the

surrounding air pressure


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Sound waves


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Range of audible frequency The range of frequency that a human ear can detect is

from 20 Hz to 20,000 Hz


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Experiments to determine speed of soundin air Pistol method

Observer A and B are positioned at a distance s apart and with ameasuring tape, measure and record s. (must be more than 1km)

A fires a starting pistol

B starts the stopwatch on seeing the flash of the pistol and stops thestopwatch when he hears the sound

The time t, is recorded The speed of sound v can be calculated by

Speed = distance / time

For better accuracy, the experiment should be repeated and theaverage speed of sound can be calculated.

The experiment can be repeated by interchanging the positions ofA and B so as to minimise the effect of the wind direction.


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Experiments to determine speed of soundin air Echo method

Observer A and B are positioned at a distance s from the wall and with ameasuring tape, measure and record s

A claps two wooden blocks.

On hearing the echo (reflected from the wall), he repeats the clap B starts the stopwatch and also starts counting from zero till the nth clap. The time interval t n is recorded The average time between successive claps is t = t n/n The speed of sound v can be calculated by

speed = distance/time


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Reflection of sound An echo is a reflection of sound Reverberation is the effect of a prolonged sound due to

the merging of many echoes

Echoes are used in determining the depth of sea andlocations of shoals of fish

nature waves

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