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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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