Reflection, Refraction And Diffraction In Waves: The travelling water waves are reflected according to the laws of reflection of light waves. To observe the reflection of water waves, straight waves are generated in the ripple tank.
Reflection
Place a plane surface obstacle in the path of waves making certain angle with the direction of propagation of waves. After striking the obstacle, the waves will be reflected in a particular direction. (Fig. 12.12-a). By using stroboscope and protractor measure the angle <i’ between the incident ray and plane obstacle. Similarly, the <r’ between refracted ray and the plane obstacle is also measured.
We will find that these two angles are equal, i.e. <i’ = <r’. In Fig 12.12 (a), the line AO drawn perpendicular to the incident waves represents the direction of propagation of incident waves. The angle between the line AO and normal NO to the plane obstacle gives the angle of incidence <i. Similarly, the line OB drawn perpendicular to the direction of propagation of reflected waves shows the direction of propagation of reflected waves and the angle between ON and OB is the angle of reflection <r. We know that angle between two line segments is equal to the angle between their respective normal’s. So <i’ = <i and <r’ = <r. According to the measurement <i’ = <r’, therefore <i = <r.
Thus we can say that the law of reflection of water waves is identical to law of reflection of light.
Refraction
When a ray of light enters from one transparent medium to another transparent medium, it bends away from its path (Fig 12.13). This bending of waves from their incident path is called “Refraction”. The refraction occurs due to the change in speed of light in the other medium. Remember that speed of light is more in air (or vacuum) than in glass or water.
Like light, water waves exhibition the phenomenon of refraction. The speed of water waves depends upon the depth of water. Its speed is reduced when it enters in shallow water.
To observe the relation between wave speeds with water depth, we cover half of the bottom of the ripple tank with a thick glass plate. In this way we get two portions of water with different depths. Remember that the edge of the plate separating these two portions should be parallel to the bar of the vibrator. When the electric motor is put on, straight waves are produced on the water surface (Fig 12.14-a). You can see in this figure that the wavelength of the waves decreases when they reach the shallow part. The frequency of the waves can not be changed in both parts of the water because it is equal to the equation v = f the decrease in wavelength is due to decrease in the speed of the waves.
For observing the refraction of water waves, we repeat this experiment in such a way that in a ripple tank the line separating two different parts of depths makes an angle with the waves. Now you will see that in addition to the change in wavelength, the waves change the direction of propagation also, as they cross the line separating the two sections of different paths (Fig. 12.14-b). In this figure we can observe that when water waves enter the shallow part they bend towards normal on the line separating the two parts. This changing of path of water waves is called refraction.
Diffraction:
Generate the straight waves in a ripple tank and place two obstacles in a line in such a way, that separation between them is equal to the wavelength of water wave. After passing through the slit between two obstacles, the waves can be noted spreading in every direction and changing to circular form (Fig. 12.15-a). This bending of waves around the corners or obstacles is called “Diffraction”.
Diffraction of waves can only be observed clearly if the size of the slit or obstacle is nearly equal to the wavelength of the wave. Fig. 12.15 (b) shows a wave passing through a slit with size larger than the wavelength of the wave. There the diffraction is not significant effect appears near the corners of the obstacle.