• 2 clear plastic cups

• 1 liter of tonic water

• Tap water (enough to fill 1 cup)

• Permanent marker

• Black paper or felt

• Clear plastic

• Sunny day

1. Mark the plastic cups "tap water" and "tonic water."
2. Fill the cups almost to the top; one with tonic water and the other with tap water.
3. Set the cups outside in direct sunlight. The best time is in the middle of the day, when the sun is closest to Earth.
4. Hold the black paper or felt behind the cups, and look across the top.
5. Do both cups of water appear the same? Do you see a blue glow on top of the tonic water? What does the black backdrop do?

Want to Learn More? Read an article related to this activity:

• The Reflection of Light

• The Refraction of Light

To find this experiment and many more please download the Lighten Up! Discovering the Science of Light book, developed through a partnership with the Optical Society of America’s Foundation (OSAF) and the Girl Scouts of the USA.