HOW COME? Shedding light on water's dark properties

Why does sand at the beach turn dark when water comes in?, a reader asks A white sand beach, blindingly bright on a July afternoon, can turn a muddy brown when the tide rolls in. Park your beach towel too close to the water, and a damp (dark) towel awaits you after a shell-picking stroll.

Of course, your swimsuit also darkens when you wade into the water. And your hair turns a deeper color when you plunge in for a swim. But although ocean water is salty (and full of floating plant-y bits), clear water works just as well, turning a white paper towel gray as you wet it to wipe up a spill.

How come? When an object looks darker, it means that less than the usual amount of light is rebounding back to us.

When light strikes a dry beach towel, some of it is absorbed, some of it is scattered in every direction from the irregular surface, and some is simply reflected back to our eyes.

But when the towel gets drenched by the ocean, water seeps in and fills the tiny airy gaps between the woven cotton fibers. Water's density is closer to that of cotton than it is to thin air. Light that was bent (refracted) at the interface between air pockets and cotton threads has smoother sailing as it penetrates wet material.

The irregular surface that scattered light when dry is actually made smoother by the film of water. More light penetrates into the fabric. And since less light reaches our eyes, the beach towel appears darker, and its colors deeper.

(On the other hand, pick up a colorful shell or stone under a few inches of seawater, and once home and dry, the colors faded, your find may look sadly nondescript.)

Similarly, light that passes into a layer of water on beachy-damp hair reflects within the layer like a Ping-Pong ball. More of the light ends up being absorbed rather than reflected, and your hair looks a shade or two darker.

According to physicist Jearl Walker of Cleveland State University, when sand grains are dry, little light is scattered or absorbed; the sand reflects brilliant light back to our eyes.

But wet sand is a different story. Sunlight is actually scattered more by wet sand than dry. But instead of being scattered outward, the light tends to be scattered inward, into the sand pile.

Since less sunlight reaches our eyes, we see the beach turn brown.

A wet fabric can also become more transparent. Instead of being reflected, more light travels into and through the material. Which is how wet clothing can turn not just dark, but embarrassing, as light bounces off skin and back out.