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Transparent and White Water
Name: Rishabh
Status: student
Grade: 9-12
Country: India
Date: Fall 2011
Question:
When the water falls slowly from a height, we can see that it is transparent, for e.g. when we pour water from one beaker to another beaker very slowly. But when water falls from a height with force, as in, a waterfall, from the tap, which is in high velocity,
>we see that water color changes to white. Why does this change in color happen?
Replies:
Rishab-
White color is our perception of near-100% reflection,
but with the wave fronts and rays all messed up.
"Scattered" is the word for that messiness.
100% reflection without messiness gives a mirrorlike impression,
and it's called "specular" reflection instead of scattered.
The reason I say all that is because, usually, any clear substance all broken up will look white.
In the waterfall, the clear water naturally breaks up into separate droplets while it is still falling,
and when it hits the bottom, it smashes into many more and smaller droplets,
before finally coagulating into the pool at the bottom.
While broken up the appearance is white.
After it coagulates again it no longer has many surfaces
so it returns to being clear.
Any known clear substance has an index of refraction greater than 1.0.
That means two effects on light:
1) some light gets reflected at each surface, such as entering and exiting a droplet.
It is often only a few percent per interface, but with many interfaces
it adds up and approaches 100%
2) the transmitted light will often be bent, prism-like.
With many droplets much of the light gets bent around as much as 180 degrees
and redirected in all directions. This too creates white appearance.
Jim Swenson
The water falling or stirring does not change color from transparent
to white. Rather under high speed conditions -- whatever the mechanical
process -- two processes can occur, scattering of light from particles of
colorless "clear" water and/or scattering of light from entrapped
particles of air.
The light scatters in all directions. The scattering in all directions
accounts for the apparent "white" color. There are a number of examples of
this common behavior. Boil a pot of water and watch the clear water scatter
light and you see the plume of "steam", which appears "white". Look up!
Notice that under the correct conditions clouds appear "white". Other colors
also occur depending upon the angle of the incident light (sunsets and
sunrises), storm clouds change from white to a dark bluish shade. All of
these and numerous other examples are due to light scattering. So it is
light scattering, instead of a change of "color" that is responsible.
Vince Calder
Rishabh
When water flows in a state we call laminar flow, like when being poured
from a beaker or coming from a garden hose, there are only two light
reflecting surfaces (looking at it in only two dimensions).
When water flows in a turbulent flow, like falling over a water fall or
spraying out of a fountain there are thousands and thousands more light
reflecting surfaces which gives the appearance of the water being white in
daylight.
This phenomenon of turbulent flow is capitalized on by fountains that spray
water into the air in colored light. This is how the display "dancing
waters" is achieved at hotels, casinos, and other tourist venues.
Sincere regards,
Mike Stewart
Hi Rishabh
yes, it looks whitish because the turmoil caused by the air bubbles in and out so fast!
Mabel
Dr. Mabel Rodrigues
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Update: June 2012
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