In physics
and engineering, a ripple
tank is a shallow glass tank of water used in schools and colleges to
demonstrate the basic properties of waves. It is a specialized form of
a WAVE TANK. The ripple tank is usually illuminated from above, so that the light
shines through the water. The ripples on the water show up as shadows on the
screen underneath the tank. All the basic properties of any waves as Light,
Electromagnetic Waves, Sound or other, including
plane waves,
reflection,
refraction,
interference and
diffraction, can be
demonstrated.
Ripples are generated by use of Vibration Generator
driven by an Wave Generator as Wave-Lab or similar.
When the rippler is lowered so that it just
touches the surface of the water, plane waves will seen to be produced.
(In the illustration, the brown rectangle is the rippler).
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Images of
plane waves. |
Demonstrating Reflection and Focusing of
Mirrors
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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.
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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.
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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.
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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.
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If the water is too shallow, viscous
drag effects cause the ripples to disappear very quickly.
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The glass should have smooth edges to minimise reflections at the edge.
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If a small obstacle is placed in the path of the ripples, and a slow
frequency is used, there is no shadow area as the ripples refract around it, as
shown below on the left. A faster frequency may result in a shadow, as shown
below on the right. If a large obstacle is placed in the tank a shadow area will
probably be observed.
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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 that the size of the
obstacle is comparable to the wavelength of the ripples.
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See also:
Huygen's
principle
Diffraction from a grid
A phenomenon identical to the x-ray diffraction of
x-rays from an
atomic crystal lattice can also
be seen, thus demonstrating the principles of crystallography. If one
lowers a grid of obstacles into the water, with the spacing between the
obstacles roughly corresponding to the wavelength of the water waves, one will
see diffraction from the grid. At certain angles between the grid and the
oncoming waves, the waves will appear to reflect off the grid; at other angles,
the waves will pass through. Similarly, if the frequency (wavelength) of the
waves is altered, the waves will also alternately pass through or be reflected,
depending on the precise relationship between spacing, orientation and
wavelength.
Interference can be produced by the use of two dippers that are attached to
the main ripple bar. In the diagrams below on the left 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. To the right is a photograph of two-point interference
generated in a circular ripple tank.
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