Detecting Ultraviolet Light Using Tonic Water

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

Background

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

• Physical Science, Transfer of Energy, Grades 5 to 8, p. 155, Item #6: "The sun is a major source of energy for changes on the earth's surface. The sun loses energy by emitting light. A tiny fraction of that light reaches the earth, transferring energy from the sun to the earth. The sun's energy arrives as light with a range of wavelengths, consisting of visible light, infrared, and ultraviolet radiation."
  
  
• Physical Science, Interactions of Energy and Matter, Grades 9 to 12, pg. 180, Item #1: "Waves, including sound and seismic waves, waves on water, and light waves, have energy and can transfer energy when they interact with matter."
  
  
• Physical Science, Interactions of Matter and Energy, Grades 9 to 12, pg. 180, Item #2: "Electromagnetic waves result when a charged object is accelerated or decelerated. Electromagnetic waves include radio waves (the longest wavelength), microwaves, infrared radiation (radiant heat), visible light, ultraviolet radiation, x-rays, and gamma rays. The energy of electromagnetic waves is carried in packets whose magnitude is inversely proportional to the wavelength."
  

Benchmarks for Science Literacy, Project 2061, AAAS

• The Physical Setting, Motion, Grades 6 to 8, pg. 90, Item #1: "Light from the sun is made up of a mixture of many different colors of light, even though to the eye the light looks almost white. Other things that give off or reflect light have a different mix of colors."
  
  
• The Physical Setting, Motion, Grades 6 to 8, pg. 90, Item #5: "Human eyes respond to only a narrow range of wavelengths of electromagnetic radiation - visible light. Differences of wavelength within that range are perceived as differences in color."
  
  
• The Physical Setting, Motion, Grades 9 to 12, pg. 92, Item #6: "Waves can superpose on one another, bend around corners, reflect off surfaces, be absorbed by materials they enter, and change direction when entering a new material. All these effects vary with wavelength. The energy of waves (like any form of energy) can be changed into other forms of energy."

Grade Level/Time

• Grade level: 5 to 9
  
  
• Time:
  
  
  • Demonstration: 20 minutes (Be sure that all students get to observe the phenomena up close)
    
    
  • Discussion: 20-30 minutes
    
    
  • Extension: 20 minutes or longer if presented as an inquiry lab
    
    

Materials

• 2 clear, plastic cups
  
  
• Permanent markers
  
  
• One liter of tonic water
  
  
• One liter of tap water
  
  
• Black paper, felt, or cloth (approximately 8" x 11")
  
  
• Sunlight (The most dramatic results will occur around noon when the sun is directly overhead. In some parts of the country with cloudy winters, you will get better results doing this activity in the months closest to summer. Results will vary according to the position of the sun and the time of day)
  
  
• For days of inclement weather use a quartz 300-watt halogen light for UV light or a black light that produces UV radiation. A slide projector using a 250 - 300 watt halogen light could provide a good alternative source of light.
  
  

Procedure

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
   

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

• This is a simple demonstration, and only informal assessment through observation and questioning would be appropriate. You may wish to explore in more detail the student's understanding of when during the day the glow should be brightest through individual questions. Remember that many students believe that the sun is always most directly overhead at noon by the clock, and have little conception of 'solar noon' which changes with the seasons.
  

Modifications for Alternative Learners

• None needed, other than allowing English Language Limited students to make oral answers as appropriate.