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 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:

Plane waves

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)

Reflection

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.

Refraction

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.

Diffraction

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.

[Diffraction.png] 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.