Violet Shines Blue
Product Manager for Physical Science, Physics, and Earth Science
Fluorescence is the property of absorbing light at a shorter wavelength and emitting light at a longer wavelength. Quinine is a fluorescent substance. In addition to providing the distinct bitter taste of tonic water, quinine naturally occurs in cinchona tree bark. Most are familiar with quinine as a treatment (but not a cure) for malaria.
Quinine absorbs invisible UV light (short wavelength) and emits it as visible blue light (long wavelength). In this activity, the violet laser provides the UV light source.
If we can’t see UV light, why is the laser beam violet in color? Wavelengths are measured in nanometers (nm), or 1 billionth of a meter. The range for UV light ends around 400 nm. The range for the color violet, which we can see, ranges from 400 to 430 nm. A violet laser pointer marked with a wavelength of 405 nm has a beam that is violet in color and that emits some UV light at the same time.
- Violet Laser Pointer, 405 nm (item #755428)
- 3 Small, Clear Plastic Cups or Glasses
- Tonic Water (regular or diet)
- Clear Sparkling Water
- Table Salt (sodium chloride)
Ensure that students understand and adhere to safe laboratory practices when performing any activity in the classroom or lab. Model proper laboratory safety practices for your students and require them to adhere to all laboratory safety rules.
Know and follow all school district guidelines for classroom laser use. Avoid direct eye exposure to laser light. Never look directly at a working laser. Never direct or deflect a laser at oneself or another person. This activity does not require the use of mirrors or highly reflective surfaces.
Teacher’s Note: Part 1 may be performed with unopened bottles of sparkling water and tonic water.
By using 2 carbonated solutions, students are less likely to form the misconception that the bubbles or carbonation contribute to the creation of the blue light.
- Fill a cup ½ full of sparkling water, then fill another cup ½ full with tonic water.
- Darken the room (complete darkness is not necessary).
- To make this a discrepant event, fill the cups before students arrive, and keep the bottles out of sight.
- Shine the laser through the cup with sparkling water, then shine the laser through the cup with the tonic water. Repeat. Only the cup with the tonic water will have a blue line.
Quinine absorbs the energy from the near UV wavelength light the violet laser produces. This causes electrons within the quinine to jump to a higher state. When those electrons transition back from their excited state to a ground state, they emit a visible blue light.
- In front of the students, fill a third cup ½ full of tonic water. Add a teaspoon of salt. Stir to dissolve.
- Shine the laser through this cup. There should be no visible line.
- Ask students to explain what happened. Responses may include:
- Reacted with quinine to form a new compound that does not fluoresce
- Disrupted the reaction between the laser and the quinine
- Destroyed the quinine
- Turned the tonic water to regular water
The addition of the sodium quenches the fluorescence of the quinine. “Quenching” refers to any process that decreases the fluorescent intensity of a substance. The chloride ion is a known chemical quencher of quinine.