Detecting Ultraviolet Light Using Tonic Water

This activity is a simple method for demonstrating UV light presence.


The energy from the sun includes not only visible light but also wavelengths longer (infrared) and shorter (ultraviolet) than visible light. The wavelengths of visible light increase from violet to red across the spectrum. Shorter than violet are wavelengths referred to as ultraviolet (UV). Ultra means beyond, so ultraviolet means beyond (actually, shorter than) violet.

The amount of UV radiation reaching the earth's surface at a particular point depends on the distance it travels through the atmosphere. During morning hours, UV radiation must travel through more of the earth's atmosphere because the sun is lower on the horizon. At noon the rays travel a shorter distance through the atmosphere because the sun is more directly overhead

The amount of UV radiation in both the stratosphere and the troposphere is an important concept.

This activity is a simple method for demonstrating UV light presence. When a photon of UV energy is absorbed, it is reemitted by the quinine in tonic water as a photon of visible light. This process is called fluorescence. The extent of fluorescence that occurs is related to the amount of UV light resulting from the angle of the sun (time of day and season of year).

Learning Goals

  1. Students will understand that UV radiation is part of the sun's electrogmagnetic spectrum; it has shorter wavelengths than visible light.

  2. Students will be able to explain the concept of fluorescence and that UV causes fluorescence in some substances such as tonic water.

Alignment to National Standards

National Science Education Standards

Benchmarks for Science Literacy, Project 2061, AAAS

Grade Level/Time



  1. Label plastic cups "tonic water" and "tap water."

  2. Fill the tonic water cup almost to the brim.

  3. Fill the tap water cup almost to the brim.

  4. Place the cups outdoors so that direct sunlight strikes the surface of the liquid in both cups.

  5. Hold a piece of paper or cloth behind the cups. Look across the surface of the tonic water and tap water through the sides of the glasses

Observations and Questions

  1. Looking at the top quarter-inch of the liquids, what do you see? (The upper quarter-inch of the tonic water cup should "glow" blue.)

  2. Did both liquids appear the same? (No, the tap water should show no change.)

  3. What effect does the black cloth or paper have on your observation? (The black cloth increases the contrast, which makes the glow of blue easier to see.)

  4. What is contained in the sunlight that causes the observed results? (Ultraviolet radiation.)

  5. Give an explanation for the observed difference between the tonic water and the tap water. (There must be a difference between the tonic water and the tap water. Teacher can explain the presence of the quinine during post-lab discussion.)

  6. Have you observed similar occurrences in other materials? (Answers will vary. Some students might be aware of the fluorescence of minerals under UV light.)

  7. How might the position of the sun affect your results? (The higher the sun is in the sky, the shorter the path length through the atmosphere (ozone layer), allowing more ultraviolet to get through.)

Extension: To demonstrate that UV light is causing the blue glow, place a Plexiglas sheet between the sun and the glass of tonic water. The Plexiglas should absorb some of the UV, preventing the fluorescence of the quinine. Do glass and Plexiglas have the same effect? What effect do suncreens have? Try coating some sunscreen (maybe use varying strengths?) on a thin glass or Plexiglas filter.

Your students could do a full inquiry lab with cellulose acetate (CA) plastic sheets (some overhead transparencies may be cellulose acetate - check the box). The CA will transmit UV, unlike glass or Plexiglass, so it can be used as a backing for various strengths of sunscreens. Students could design their own studies and collect qualitative data on the amount of blue glow associated with different sunscreens or tanning creams.

You may want to consider doing this activity at different times of the day so that students can compare the differences.

Assessment Ideas

Modifications for Alternative Learners

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