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Black Light Basics

In this activity, students examine a variety of materials to determine whether they contain phosphors and relate the activity to forensics. 

A black light gives off harmless, highly energetic, ultraviolet (UV) light that is invisible to humans. Certain fluorescent substances absorb ultraviolet light and re-emit it at a different wavelength, making the light visible and the material appear to glow. Forensic scientists use ultraviolet lights at crime scenes to identify materials, based on our knowledge of what materials contain these fluorescent substances. 

What you see glowing under a black light are phosphors.

A phosphor is any substance that emits visible light in response to some sort of radiation. A phosphor converts the energy in the UV radiation from a black light into visible light. For example, body fluids like blood and urine contain these fluorescent molecules and are made visible with the help of a black light. Black lights can also be used to discover counterfeit notes that don’t contain the fluorescent symbols included in legal banknotes. 

Common materials that fluoresce under black light are: 

  • White paper treated with fluorescent compounds to help it appear brighter and therefore whiter. White paper made after 1950 contains fluorescent chemicals while older paper does not. Sometimes forgery of historical documents can be detected by placing the documents under a black light to see whether or not they fluoresce. 
  • Quinine, the bitter flavouring in tonic water, which glows blue-white when placed under a black light.
  • Some of the whiteners in detergent that make your T-shirt look “whiter than white.” 
  • Pens containing fluorescent ink, used to “invisibly” mark items. If the marked objects are stolen, a black light can be used to search for these security markings.
  • Petroleum jelly, such as Vaseline, which glows bright blue under a black light. 
  • Ripe bananas glow fluorescent blue under a black or ultraviolet lamp. It is thought that ripe bananas fluoresce in order to attract animals that do not see in the “normal” light range, such as fruit bats.

Objectives

  • Predict, compare and explain why some objects fluoresce under UV (black) light.

Materials

  • Per Class or Group:
    black light (Rona, Home Depot, Canadian Tire, or Teacher Source)
    various materials to test

Key Questions

  • Why do some objects fluoresce?
  • Why is black light invisible?
  • Why are black lights used in forensic science?

What To Do

Set up

  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
  1. 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.
  2. 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.
  3. 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?