Light fantastic

Includes: Indoors, Outdoors, Technology and science

Activity time:
< 10 mins


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All aglow

Learn about the action of ultra-violet light.


  1. Fill one plastic glass with tonic water and one with tap water almost to the brim and label them.
  2. Place the glasses in direct sunlight. (For best results, do this experiment in the middle of the day.
  3. Hold a piece of black paper or cloth behind the cups. Look across the surfaces of the tonic water and tap water through the sides of the glasses. You should see a blue glow on the surface of the tonic water (but not the tap water) from the ultra-violet light in sunlight.

When ultra-violet light is absorbed by the quinine in tonic water, it appears again as visible light (the blue fluorescence).

Planning checklist

Safety notes

Use sunblock if you do this activity outside on a sunny day.

Section Number of girls per adult
Rainbows 5
Brownies 8
Guides 12

How many adults are needed?


2 adults needed