Does water reflect or refract light?

Answered by Alana Jensen, INL ESER Program

Water is both a reflector and refractor of light. Let’s examine these properties of light separately.

Reflection: Reflection is when light hits the surface of an object and then that light travels to our eyes so we can see. All materials reflect light to some extent, but a rough surface scatters the reflected rays in all directions, so reflected images are blurred beyond recognition. Water is much flatter and smoother than most surfaces. With a very smooth surface, all the reflected light rays stay arranged in the same way they were arranged before hitting the surface (except for being flipped into a mirror image, of course).

Refraction: When light travels from one transparent medium (air) to another transparent medium (water), the speed of the light slows down and when it hits water it changes its direction slightly. This change in the direction of light is known as refraction of light. In simple language, light refraction is the bending of light. You can see light refracting in water with a piece of paper, a clear glass of water and a marker. Fill the glass with clear water. Draw an arrow of the piece of paper. Place the water filled glass in front of the arrow on the paper and adjust the paper until you can see the arrow through the glass. Lower the paper until the arrow is below the surface of the water. Did your arrow change direction? As light waves traveled from your arrow into the water, through the water, and finally out of the water into the air again, the waves refracted (bent) and the arrow looks reversed.

It’s Idaho Water Awareness Week!
Check out some fun activities and videos at www.eieea.org/waterfestival/

Write a Secret Message Using Water’s Reflection Properties
Materials: Two sheets of paper, towel, ballpoint pen, bowl or tray of water

  1. Put one sheet of paper in the water and leave it to soak a few minutes.
  2. Pull the wet paper from the water, holding it above the bowl or tray to allow the extra water to drip back into the container.
  3. Place the wet paper on a table or desk, making sure it’s nice and flat.
  4. Gently pat the paper dry with a towel.
  5. Put the dry sheet of paper on top of the wet sheet and quickly start writing your secret message on it. Hurry! If you wait too long, the top sheet of paper will get wet and start to rip!
  6. Take the top sheet of paper away. You can destroy it so no one can read your secret message.
  7. Hold the wet paper up to a light. Can you see your message?
  8. Let the wet piece of paper dry.
  9. When it’s completely dry, look at the paper. Can you read the message now?
  10. To reveal the message, wet the paper and hold it up to a light like you did before. Your message will appear!

What’s happening?
Paper is made from trees. Trees are chopped into tiny pieces, releasing the fibers within. Then fibers are then made into a wet, soupy mixture that is rolled, squeezed and dried. The fibers in the paper get tangled and stick together.

Micrograph of tissue paper.
By Richard Wheeler (Zephyris) – Own work, CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=10688563

The tangled fibers are what makes paper strong. When paper gets wet, the fibers unstick from each other a bit, which is why wet paper tears easily.

In this activity, you’re squeezing the wet paper between a table and a pen. The wet paper fibers are free to move just enough to thin the paper in those areas. If you look closely, even at the dry sheet, you might be able to see where the paper has thinned.

When the paper dries, air is trapped in gaps between the fibers causing light to reflect off the paper, making it appear opaque (not see-through). When water fills the gaps, less of the light is reflected and more passes through the paper, so it looks see-through.

It’s the combination of thin paper and water that work together to leave a watermark – a secret message that can only be revealed with water.

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