This is a How-to about a Ripple tank. A ripple tank is a shallow glass tank of water used in schools and colleges to demonstrate the basic properties of waves. The tank is usually illuminated from above, so that the light shines through the water. (Some small ripple tanks fit onto the top of an overhead projector i.e. they are illuminated from below.) The ripples on the water show up as shadows on the screen underneath the tank. All the basic properties of waves can be demonstrated.
A piece of wood is suspended above the tank on elastic bands so that it is just touching the surface. Screwed to wood is a motor that has an off centre weight attached to the axle. As the axle rotates the motor wobbles.
Properties of waves that can be demonstrated with a ripple tank:
The ripple motor is connected to a variable 6V DC supply. The rippler is lowered so that it just touches the surface of the water and the motor is turned on. Plane waves will move towards the left.(The brown rectangle is the rippler)
Demonstrating Reflection and Focusing of Mirrors
By placing a metal bar in the tank and tapping the wooden bar a pulse of three of four ripples can be sent towards the metal bar. The ripples reflect from the bar. If the bar is placed at an angle to the wavefront the reflected waves can be seen to obey the law of reflection. The angle of incidence and angle of reflection will be the same.
If a concave semicircular obstacle is used, a plane wave pulse will converge on a point after reflection. This point is the focal point of the mirror. Circular waves can be produced by dropping a single drop of water into the ripple tank. If this is done at the focal point of the "mirror" plane waves will be reflected back.
If a sheet of glass is placed in the tank the depth of water in the tank will be shallower over the glass than elsewhere. The speed of a wave in water depends on the depth, so the ripples slow down as they pass over the glass. This causes the wavelength to decrease. If the junction between the deep and shallow water is at an angle to the wavefront , the waves will refract. In the diagram above, the waves can be seen to bend towards the normal. The normal is shown as a dotted line. The dashed line is the direction that the waves would travel if they had not met the angled piece of glass.
In practice, showing refraction with a ripple tank is quite tricky to do.
- The sheet of glass needs to be quite thick, with the water over it as shallow as possible. This maximizes the depth difference and so causes a greater velocity difference and therefore greater angle.
- If the water is too shallow viscous drag effects cause the ripples to disappear very quickly.
- The glass should have smooth edges to minimise reflections at the edge.
If a small obstacle is placed in the path of the ripples there is no shadow area as the ripples refract around it. If a large obstacle is placed in the tank a shadow area will be observed.
If an obstacle with a small gap is placed in the tank the ripples emerge in an almost semicircular pattern. If the gap is large however, the diffraction is much more limited.
Small in this context means comparable to the wavelength of the ripples.
Interference can be produced by the use of two dippers that are attached to the main ripple bar. In the diagram below the light areas represent crests of waves, the black areas represent troughs. Notice the grey areas: they are areas of destructive interference where the waves from the two sources cancel one another out.