What is a wave? You will know many waves but it is not easy to define what a wave is.
“ A method by which energy is transferred” is the closest definition we can provide.
If you consider kinetic energy in the molecules, (heat) whether it is from a flame or the sun, there are only three methods by which energy can spread. They are conduction, convection and radiation. The last mentioned category,
radiation, is the transfer of energy by waves. This type of radiating waves are called
electromagnetic waves. The waves outside this category are ‘mechanical waves’ That’s all you get in the whole of the Universe.
Fig.1 Waving ribbons.
Normally a wave diagram is a graphical representation of shifting of particles in a wave. Displacement plotted against time.
Experiment
Take a marker pen placing it on paper and move the hand up and down. What will you get ? You will get a vertical line. On the same line you will be drawing again and again. Now get a friend to pull the paper to a side while you are drawing. What will you get ?
Fig.2 This is what we got.
Let us learn the parts and the working of a wave.
The ‘too and fro movement’ of the marker is called oscillations or vibrations. When the marker goes up from the resting position, comes back and goes deep down and gets back, it completes one cycle. By this time a crest and a trough will form.
The number of such cycles form during one second is called frequency. The unit for this is Hertz, ( Hz), The wave length is the distance the wave travels for one complete cycle. The furthest displacement of the marker from the resting position is the amplitude
Fig. 3 Naming the pasts of a wave.
The terms are again explained here.
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Amplitude:
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The maximum displacement of a particle from the normal position. This increases with the increase of energy being carried by the wave.
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Wave length:
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This is the distance the wave will go during one complete oscillation or a cycle. ( A crest and a trough creates a complete cycle)
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The distance between the two nearest particles at the same state of motion.
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Frequency:
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The number of oscillations in one second.
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Velocity:
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As the frequency gives the number of oscillations per second, and the wave length gives the distance travelled for one oscillation what will the product of frequency and wave length give?.
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1.0 Electromagnetic waves.
i..According to the modern theories, there cannot be anything moving faster than these waves. Their speed in space is 300,000,000 meters per second (3x 108).
ii. They are caused by fluctuation of an electric field perpendicular to a magnetic field and perpendicular to the direction of travel. These really are transverse waves.
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Type Frequency Range: Wavelength Range:
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Radio Waves:
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<3x1011 Hz
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>1mm
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Microwaves:
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3x1011 - 1013 Hz
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1mm - 25um
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Infrared:
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1x1013 - 4x1014 Hz
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25um - 750nm
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Light:
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4x1014 - 7.5x1014 Hz
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750nm - 400 nm
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Ultraviolet:
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1015 - 1017 Hz
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400nm - 1nm
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X-rays:
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1017 - 1020 Hz
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1nm - 1pm
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Gamma Rays:
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1020 - 1024 Hz
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<10-12 m
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Chart: Electromagnetic spectrum.
Fig.4.Light consists of transverse waves . Fluctuating of electric and magnetic fields.
2.0 Mechanical waves.
These waves can travel only in a material medium. There are three kinds of such waves.
i. Longitudinal waves. Ii. Transverse waves iii. Surface waves.
i. Longitudinal waves:
These can travel in solids, liquids or gases. Particle show only to and fro movement parallel to the direction of travel. Energy is transferred with the wave.
Sound is a very good example of this.
Fig. 5 Spreading of a pressure wave.
The Fig 5 shows how a sound wave is spreading all around in the form of spheres. A vibrating object such as a drum skin or a loud speaker cone will cause very slight changes in air pressure. When the pressure increases we call it a ‘compression’. (Sound waves are often compared to ripples in water which is not correct. They are surface waves) When the pressure decreases it would be a ‘rarefaction’. Air molecules are denoted by curved lines here.
It is important to note that sound can travel, even in solids and liquids only as longitudinal waves.
A sound wave has a speed of 340ms-1 in air under normal conditions. 1,484 m/s in water and 0ver 5000 m/s rigid solids.
Fig. 6. A very thin strip of a pressure wave in air. .
A vertical line stands for a row of molecules in air. As the cone of the speaker gets pushed out a compression results. The first compression is labelled as C 1. The rows below indicate how it travels to the right.
This can be converted to the wave form by drawing a graph. The graph below shows how a single molecule moves . The displacement is given by x axis while time is kept by y axis.
2. Transverse waves.
Take a garden hose or a flexible current cable. Lay it in the ground. Just give a jerk by lifting the end quickly. You will see a traveling wave pulse. It would be only a crest traveling. Now you can hold the end firmly and swing it to and fro in a horizontal plane. You will see complete waves traveling. These are transverse waves.
This type of a wave is possible only in solids. The particles vibrate perpendicular to the direction of the wave.
The fig.7. shows a wave traveling to right.
Note that the particles move only in a vertical plane. What can you say about the motion of the particles marked a, b, c, d and e?
a. Stopped movement, about to turn direction. Maximum gravitational potential energy.
b. This has the maximum kinetic energy. Velocity up.
c. This is similar to a but trying to go up. Maximum elastic energy.
d. This also has the maximum kinetic energy moving down.
Electromagnetic waves and some seismic waves are transverse waves.
3. Surface waves
As the name suggests these can occur between two surfaces such as between ocean and sky, between different layers of the Earth or between two layers of water separated due to density. They can be considered as a mixture of Longitudinal and transverse waves. This blending results in a rolling motion of the particles.
If you watch the movement of a float due to waves it would be possible to observe that it moves in the direction of the wave with the crest and goes back with the trough. So the water moves in a circle while the wave proceeds. As the wave approaches the shore, vertical diameter decreases producing an elliptical motion .
An ocean wave normally carries the energy received from the wind.
