Now we take an over of certain properties (e.g., reflection, refraction, interference, diffraction etc.) of waves. The simple apparatus, called ripple tank is used for this purpose. The construction and use of ripple tank is being given below:
This apparatus consists of a rectangular tray with glass bottom and is placed nearly half meter above the surface of a table by means of four supporting legs. Waves are produced on the surface of water present in the tray by means of a vibrator. This vibrator is an oscillating electric motor which is fixed on a wooden plate. This plate is suspended by means of rubber bands and its lower end touches the water surface. On setting vibration on, this plate starts vibrating and straight waves are generated on the water surface (Fig. 12.8). An electric bulb is hung over the tray. With the help of its light, the image of water waves is obtained on the white paper or screen. The crests of the waves appear as bright lines on the paper because they function like convex lens and converge the rays of light falling on them. On the other hand, troughs behave like concave lenses and diverge the light, thereby appearing as dark straight portions between bright lines.
To generate circular wave, The vibrating bar is raised up and a knob attached to it is lowered in such a way that it touches the water surface. When vibrator is set on, circular waves are produced on the water surface (Fig. 12.10). Remember that Fig. 12.10 is a snapshot of such waver taken by a very high power camera; these waves are actually seen moving on the paper.
For the detailed analysis of these moving waves they should be “frozen” by one way or the other. This is done by means of a device called stroboscope. A simple stroboscope has been shown in Fig 12.11. It consists of a round disc with slits on its edge. This disk can be rotated about its centre by putting it into motion by a finger. When we see through this disc, we see waves only when a slit passes against our eye. If we adjust the speed of stroboscope, so that a slit in front of our eye is replaced by another during the time in which the waves move forward by one wavelength, the waves appear to be stationary and we say that their motion is father “frozen”.
Relation between velocity, frequency and wavelength
We can find easily the wavelength () of a wave, which is the distance between the centers of two successive crests or troughs On the screen below the tray of ripple tank we can see waves in motion for which the distance between two successive crests remains constant and can be measured easily.
If we place a piece of paper on the water surface and observe its motion, we find that it vibrates up and down on its place and do not move forward in the direction of motion of the wave. We also note that time of one vibration of the paper is equal to the time taken by two successive crests or troughs to pass through a certain point. As the distance between two successive crests or troughs is equal to one wavelength, therefore, during the time of vibration of a piece of paper the wave covers a distance equal to one wavelength. Moreover, if we measure the time of one vibration of piece of paper and time period of the plate, then we will observe that both are equal. So, if the speed of the wave is v and the time period of the plate, producing waves is ‘T’ second, then the distance covered by the wave will be vT. As it is equal to the wavelength, therefore
Equation (12.8) shows that wave speed is the product of frequency and wavelength. Frequency of waves, that is number of waves passing through a certain point in one second, can be changed by changing frequency of the vibrator. Other two quantities (ie., v and ) depend upon the properties of the medium.
Example 12.3: The wavelength of a wave passing through a medium is 0.1 meter. If wave speed is 2ms, then calculate the wave frequency.
Using ripple tank we will discuss four properties of waves.