What is a Wave? | Types of Waves

What is a Wave?

"A wave is a disturbance in a medium that transfers energy from one place to another."

Waves transfer energy over a distance. Can waves move matter the entire distance? For example, a tide can travel many kilometers. The water moves up and down- a disturbance that travels in a wave, transferring energy, not matter.

What are the types of waves?

There are the following types of waves:

        1.     Mechanical wave            2.     Electromagnetic waves

1.     Mechanical waves

• Mechanical waves are such waves that need a medium for propagation.

• Mechanical waves are produced by vibratory motion in the respective medium.

• Sound waves, water waves, and seismic waves are some mechanical examples of waves.

• Mechanical waves consist of transverse and longitudinal waves. 

• Mechanical waves cannot travel through a vacuum.

• All mechanical waves travel through their media at different speeds depending upon the physical properties of the respective medium.

2.     Electromagnetic waves

• Electromagnetic waves are such waves that do not need a medium for propagation.

• Electromagnetic waves are produced by a changing of electric and magnetic fields.

• Radio waves, microwaves, light waves, U.V waves, and infrared waves are some examples of electromagnetic waves.

• Electromagnetic waves are only comprised of a transverse wave in nature.

• Electromagnetic waves travel through the vacuum at the speed of `3\times10^8` m/s.

• All electromagnetic waves can travel through transparent media at different speeds depending upon the refractive index of the respective medium.

Properties of Waves

The depth at which the dipper is placed affects the amplitude of the waves, while the frequency of vibration of the dipper corresponds to the frequency of water waves produced.

Let us demonstrate some wave properties such as reflection, refraction, and diffraction concerning the ripple tank

Reflection of the waves

Figure 1.11 shows how one can demonstrate the reflection of the waves? When a vertical straight surface is placed in the path of the incoming waves. The incident waves are reflected from the surface at the same angle. It can be seen that the reflected waves obey the law of reflection.

For Example, the angle of the incident wave along the normal will be equal to the angle of the reflected wave. Hence we define the reflection of waves as:

Definition: "Bouncing back of waves into the same medium by striking other medium surface is called reflection".

Refraction of waves

Figure 1.12 shows how refraction of the waves can be demonstrated. When a flat piece of a block is immersed in the ripple tank, water depth becomes shallow. You will find that the wavelength of the plane waves shortens and changes direction

Fig. 1.13 as they move from the boundary between two media, deep to shallow water. However, the frequency of water waves stays the same in both waves because it is the same as the frequency of the vibrator. This result shows that the speed of a wave in water depends on water depth. Waves travel faster in deep water than in shallow water. This effect is called refraction. Hence we can define the refraction of the waves as: 

Definition: "When a wave enters from a region of deep water to a region of shallow water at an angle, the wave will change its direction".

Diffraction of waves

Figure 1.14 shows when an obstruction or a straight surface with a gap in the ripple tank is placed in the path of the incoming water waves, they strike it, the waves will bend around the sides of an obstruction or spread out as they pass through a gap. This phenomenon is called diffraction.

Diffraction is only significant if the size of the gap is about the same as the wavelength of the incident wave, narrows the gap whose width is equal to the wavelength of the incoming ripples, the ripples that pass through the gap are almost circular and seem to originate from a point source situated in the gap. Wider gaps produce less diffraction. Hence we define the diffraction of waves as:

Definition: "The spreading of the waves near an obstacle is called Diffraction"

Waves Characteristics

The following are some terms used to describe wave motion.

Time-Period (T), is the time taken for any one point on the wave to complete one oscillation.

The SI unit of the period is second (s).

Frequency (f), is the number of complete waves produced by a source per unit of time.

Thus, 

`Frequency =` Number of complete waves produced `/ ` time taken

If the number of waves produced =  is 1 

And time is taken `= T` Then `f =` `frac(1)(T)` 

In general,

"Frequency is also defined as the reciprocal of the period".

The SI unit of frequency is the hertz (Hz). 

Wavelength (`lambda`), is the linear distance between two successive crests or troughs in a transverse wave and two successive compressions and rarefactions in a longitudinal wave. 

Its SI unit is meter (m)

Wave speed (v), is the speed at which a wave travels.

"It is defined as the distance traveled by a given point on the wave, such as a crest in a given interval of time".

`Speed =` `frac(Distance)(time)`  or

Let us consider a wave,

Distance travelled`=λ` and time is taken `=T`, then

`v=``frac(S)(t)`

Hence `s=λ`
so `t=T`
`v=``(λ)/(T)`     ---------(i)
`therefore` `(1)/(T)=f`
        `v=fλ`
        `S=v*t`
        `λ=v*T`             ---------(ii)

The speed of the wave can also be written as `V=fλ`

In the SI system, the wave speed is measured in `m/s`.

The Wavefront is an imaginary line on a wave that joins all points that are in the same phase.

A wavefront is usually drawn by joining all wave crests. There are two types of wavefront, depending on how the waves are produced, which are concentric circles; figure 10.16. (a) and plane straight lines; figure 10.16. (b).

Figure 10.16 (a). In a ripple tank, a dipper can produce circular waves. These waves have a circular wavefront.

Figure 10.16 (b). In a ripple tank, a plane dipper can produce plane waves. These waves have a plane wavefront.

Read more...

General Waves Properties