Black Light Investigations

Objectives

• Describe the wave-like nature of light and explain that only certain wavelengths are visible to the human eye.

Materials

• Per Demo:
  rope or slinky
  Black light (Rona, Home Depot, Canadian Tire, or Teacher Source)
  Items that glow (club soda or tonic water, laundry detergent, toothpaste, glow-in-the-dark nail polish, postage stamps, paper, paper money)

Key Questions

• What colours of light have a shorter wavelength?
• What colours of light have a longer wavelength?
• Why is black light invisible to the human eye?
• What kind of objects tend to glow under black light?
• Why do some objects glow under black light?

What To Do

Part One

1. Hold up the rope/slinky and ask for a volunteer.
2. Explain that we can think of light as a wave, with peaks and valleys. Different colours of light have different numbers of peaks and valleys.
3. Shake the rope/slinky up and down slowly. Say that red light has a long wavelength and doesn’t have many peaks and valleys.
4. Shake the rope quickly. Say that violet light has a short wavelength and has lots of peaks and valleys.
5. Shake the rope even more quickly. Say that ultraviolet light has an even shorter wavelength and that it’s invisible to us because the human eye can’t detect that wavelength.

Part two

1. Set up a central table with the black light and some materials to test. A comparison test could include the following materials:

Will Glow	Won’t Glow
Tonic water	Soda water
Laundry detergent	Flour
Recently washed white T-shirt	Colourful T-shirt
Real $20 bills	Monopoly money
White paper	Coloured paper
Ripe bananas	Green bananas
Fluorescent pen	Regular pen

2. Gather students around the table so that they can see all of the objects. Make predictions as to which materials will or won’t glow.
3. Turn off the classroom lights and pull the blinds. Test each material by placing it beneath the black light. The fluorescent materials will visibly glow.
4. Discuss why each of the test materials fluoresces and how this property could be useful in examining evidence from the scene of a crime.

Extensions

• Try using florescent powder as fingerprinting powder. After dusting the fingerprints, either shine the black light directly onto the object or lift the prints using clear tape and paste the strips of tape onto black construction paper. Can students create a crime scene in which black-light evidence can help solve the crime?
• See Black Light Basics Resource for more information of florescence.