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Purple paper with the words Double Helix glowing in the dark.
Light up the dark with Double Helix

Lots of glow-in-the dark experiments need black lights to work. In this activity, find out how to make a simple black light and create your own glow in the dark sign!

You will need

  • A mobile phone with a camera light (or a small torch)
  • Clear tape
  • Paper
  • Fluorescent highlighter (preferably yellow)
  • Blue and purple markers

What to do

  1. Covering the light on a mobile phone with tape.Tear a small piece of tape and cover the camera light on your phone. Be careful to only cover the light, not the camera lenses.
  2. Colouring in the tape with blue markerUse a blue marker to colour the area over the camera light as shown.
  3. Tear off another piece of tape and repeat steps 1 and 2.
  4. Colouring tape with a purple markerUsing a third piece of tape, cover your camera light again. This time, colour the area directly above the light with your purple marker. You’ve now made a black light!
  5. Writing a message on paper with highlighter pen.Write your message with a highlighter on a piece of plain white paper.
  6. Find a dark place and turn on your mobile phone flashlight.
  7. Message glowing in the dark.Shine your black light onto your message.

What’s happening?

Many highlighters are so bright because they are fluorescent. Fluorescence is when objects absorb specific colours and emit a different coloured light. Highlighters contain fluorescent chemicals that help them absorb both visible light and ultraviolet (UV) light. The human eye can’t see UV rays, but fluorescent chemicals in highlighters can absorb UV rays and re-emit them as visible light. This re-emitted visible light makes highlighters look like they’re glowing to our eyes.

Many glow-in-the dark experiments require black lights that emit UV rays to help us see fluorescent objects. When you colour on tape with blue and purple markers, you block out all other colours that make up the visible light, except purple and blue. Blue and purple emit light similar to UV rays. When these UV-like lights are absorbed by the fluorescent chemicals in your highlighter, your highlighter becomes extra bright in the dark by re-emitting the UV-like light as visible light! This conversion from UV to visible light is what helps your sign glow.

Now that you’ve created a black light, it might be time to create secret glow-in-the dark messages with your friends and family!

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