Ripple Tank

            The ripple tank is used to generate water waves in laboratory. It is useful in demonstrating wave properties such as reflection and refraction. It consists of a shallow tray of water with a transparent base, a light source directly above the tray and a white screen beneath the tray to capture the image of the shadows formed when water waves spread across the tank as shown above. Straight waves can be set up by using a straight dipper, while circular waves can be formed by using a spherical dipper. Both dipper are vibrated up and down by a motor.

The waves will be seen in bright and dark patches on the screen below the tray. These patches show the position of the crests and troughs of the waves. The dark patches will correspond to the crests and bright patches will be the troughs.

Properties of waves

The followings are the properties of wave; reflection, refraction, diffraction interference and polarization.

Reflection

The law of reflection of waves states that a wave will always reflect in such a way that the angle at which they approach a barrier equals the angle at which they reflect off the barrier.

In a Ripple Tank , if a linear object attached to an oscillator bobs move back and forth within the water in the tank, it becomes a source of straight waves. These straight waves have alternating crests and troughs. As viewed on the sheet of paper below the tank, the crests are the dark lines stretching across the paper and the troughs are the bright lines. These waves will travel through the water until they encounter an obstacle - such as the wall of the tank or an object placed within the water. The diagram at the right depicts a series of straight waves approaching a long barrier extending at an angle across the tank of water. The direction that these wavefronts (straight-line crests) are traveling through the water is represented by the blue arrow. The blue arrow is called a ray and is drawn perpendicular to the wavefronts. Upon reaching the barrier placed within the water, these waves bounce off the water and move in a different direction. The diagram below shows the reflected wavefronts and the reflected ray. Regardless of the angle at which the wavefronts approach the barrier, the law of reflection holds true .

Refraction

Refraction can also take place in the plane waves in the ripple tank by placing a sheet of glass in the water to make it shallower.

Reflection involves a change in direction of waves when they bounce off a barrier. Refraction of waves involves a change in the direction of waves as they pass from one medium to another. Refraction, or the bending of the path of the waves, is accompanied by a change in speed and wavelength of the waves.  

Angle A= Angle C

Angle A greater than angle B

Refractive Index

The refractive index   of medium A is defined as the ratio of the speed of wave in B to the speed of wave in A

Refractive index     

The refractive index   of medium B is defined as the ratio of the speed of wave in A to the speed of wave in B

Refractive index  

During refraction the frequency of the wave remain the same but the wavelength changes.

Refractive index

Refractive index

Snell’s Law or Law of refraction

Snell’s law is a formula used to describe the relationship between the angles of incidence and refraction; when referring to light or other waves passing through a boundary between two different isotropic media such as water, glass and air

The constant n is the indices of refraction for the corresponding media.

1. The incident ray, the refracted ray and the normal to the interface of two transparent media at the point of incidence, all lies in the same plane.
2. The ratio of sine of angle of incidence to the sine of angle of refraction is constant, for the light of a given colour and for the given pair of media.

Diffraction

Diffraction is the spreading of waves around obstacles. Diffraction takes place with sound; with electromagnetic radiation, such as light, X-rays, and gamma rays; and with very small moving particles such as atoms, neutrons, and electrons, which show wavelike properties. One consequence of diffraction is that sharp shadows are not produced.

diffraction involves a change in direction of waves as they pass through an opening or around a barrier in their path. Water waves have the ability to travel around corners, around obstacles and through openings. This ability is most obvious for water waves with longer wavelengths. Diffraction can be demonstrated by placing small barriers and obstacles in a ripple tank and observing the path of the water waves as they encounter the obstacles. The waves are seen to pass around the barrier into the regions behind it; subsequently the water behind the barrier is disturbed. The amount of diffraction (the sharpness of the bending) increases with increasing wavelength and decreases with decreasing wavelength. In fact, when the wavelength of the waves is smaller than the obstacle, no noticeable diffraction occurs.

Diffraction of water waves is observed in a harbor as waves bend around small boats and are found to disturb the water behind them. The same waves however are unable to diffract around larger boats since their wavelength is smaller than the boat. Diffraction of sound waves is commonly observed; we notice sound diffracting around corners, allowing us to hear others who are speaking to us from adjacent rooms. Many forest-dwelling birds take advantage of the diffractive ability of long-wavelength sound waves. Owls for instance are able to communicate across long distances due to the fact that their long-wavelength hoots are able to diffract around forest trees and carry farther than the short-wavelength tweets of songbirds. Diffraction is observed of light waves but only when the waves encounter obstacles with extremely small wavelengths (such as particles suspended in our atmosphere)

Interference of waves

This is a process whereby two identical waves traveling in the same direction are superimposed

When the two waves travelled the same distance or one of the waves travels a wave length further than the other and they are now in step. The two waves will combine and form a wave with twice as amplitude of the initial waves. This is additive or Constructive interference.

Destructive interference is form when one of the waves travel half wavelength than the other so that the crest of the first wave align with the trough of the second wave. The two waves cancel each other out and disappear.

Polarisation of waves

Polarization, also called wave polarization, is an expression of the orientation of the lines of electric flux in an electromagnetic field ( EM field ). Polarization can be constant -- that is, existing in a particular orientation at all times, or it can rotate with each wave cycle.

Polarization is important in wireless communications systems. The physical orientation of a wireless antenna corresponds to the polarization of the radio waves received or transmitted by that antenna. Thus, a vertical antenna receives and emits vertically polarized waves, and a horizontal antenna receives or emits horizontally polarized waves. The best short-range communications is obtained when the transmitting and receiving (source and destination) antennas have the same polarization. The least efficient short-range communications usually takes place when the two antennas are at right angles (for example, one horizontal and one vertical). Over long distances, the atmosphere can cause the polarization of a radio wave to fluctuate, so the distinction between horizontal and vertical becomes less significant.

Some wireless antennas transmit and receive EM waves whose polarization rotates 360 degrees with each complete wave cycle. This type of polarization, called elliptical or circular polarization, can be either clockwise or counterclockwise. The best communications results are obtained when the transmitting and receiving antennas have the same sense of polarization (both clockwise or both counterclockwise). The worst communications usually takes place when the two antennas radiate and receive in the opposite sense (one clockwise and the other counterclockwise).

Polarization affects the propagation of EM fields at infrared ( IR ), visible, ultraviolet ( UV ), and even X-ray wavelength s. In ordinary visible light, there are numerous wave components at random polarization angles. When such light is passed through a special filter, the filter blocks all light except that having a certain polarization. When two polarizing filters are placed so a ray of light passes through them both, the amount of light transmitted depends on the angle of the polarizing filters with respect to each other. The most light is transmitted when the two filters are oriented so they polarize light in the same direction. The least light is transmitted when the filters are oriented at right angles to each other.

The effect of polarization on visible light can be striking. Anyone who has worn polarized sunglasses, or who has used polarizing filters in photography, knows how a clear sky polarizes sunlight. Polarized sunglasses can reduce glare reflected from surfaces; this is useful under certain driving conditions and can also make it easier to see beneath the surface of a body of water. In twisted nematic display s (TN displays), polarizing filters are used in conjunction with a special liquid to brighten and darken regions of the display as external voltage s are applied. This makes it possible to display alphanumeric characters in wristwatches, cell phones, and various other consumer electronic devices.

Read more about PHYSICS for High Schools