A Splash of Color
Clearly, pure water is perfectly clear…isn’t it? Find out for yourself by filling a tall plastic tube with water.
- Clear polycarbonate fluorescent lamp tube guard, 4 feet (1 1/4 m) or longer (or other similarly sized clear plastic tube)
- Scissors
- Clear waterproof tape, such as clear packing tape, Crystal Clear Gorilla Tape, or 3M Transparent Duct Tape
- Paper cup with white interior
- White bucket, plastic bin, or large bowl
- Two-liter bottle or similarly sized pitcher
- Water, enough to fill the tube entirely (use clean tap water to start, but also feel free to experiment with other fresh or saltwater samples from natural sources)
- White paper
- If the clear plastic tube came with plastic caps on the ends, remove the caps.
- Cut off the bottom of the paper cup, then cut the bottom to fit the diameter of the plastic tube. Discard the sides of the cup.
- Use the clear plastic tape to tape the bottom of the cup to one end of the tube, making sure that the coated inside of the paper cup faces the inside of the tube (see photo below). Add tape as necessary so the tube is as watertight as possible.
- To test for leaks, place the sealed end of the tube in the bottom of the empty bucket, and use the two-liter bottle to pour water into the top. Check for water leaking out the bottom. It’s okay if there are some tiny leaks, but the tube should be able to hold the majority of the water for at least a few minutes.
Stand the tube on end in the bucket so that the open end is on top. Look down into the top of the tube, and notice the color at the bottom. The bottom of the tube should look white. Use the two-liter bottle or pitcher to fill the tube to the top with a water sample (for example, tap water, lake water, or ocean water). The water depth should be at least 3 feet (1 m).
Start by looking through the side of the tube as you hold a piece of white paper behind it. Does the water appear to have a color?
Now look down into the top of the tube toward the sealed end. Does the water appear to have a color?
If you have another water sample, repeat the steps above. Do the water samples appear the same from the side view? Do they appear the same from the top view?
Viewed from the side, the water likely looked clear and colorless. But viewed from above, you probably did see color—green or blue for tap water, and possibly yellow or brown for natural samples.
Many people are surprised to find “clear” water looking blue-green. The blue-green color that appears and strengthens with increasing depth arises from an intrinsic property of water—its tendency to absorb light at the red end of the visible spectrum.
White light is made up of all different colors of light. As white light passes through increasing depths of water, it loses more and more red light to absorption, and the remaining light looks increasingly blue-green. You might have noticed this before in a bathtub full of water.
This blue-green effect is, er, muddied if the water is turbid—that is, containing mud, algae, or other sediments. In turbid water, the light scattered off this particulate matter will overwhelm the water’s natural blueness and appear brown, yellow, or green.
Try pouring the water into the tube gradually, noticing the color at different depths. How does it change?
Drop small colored objects into the bottom of the tube. Do their colors look different underwater?
You can include an engineering challenge in this Snack by asking students to figure out how to make the tube hold water. Provide a wider selection of materials (different types of paper, tape, rubber bands, etc.), and tell students that the sealed end of the tube must be white when viewed through the top of the tube.
