Skip to content

Ultraviolet light

Radiation and the atom · level 2, lesson 2 of 6

A question to start

Under a special "black light", a white T-shirt glows blue and a highlighter pen glows bright yellow-green. Where does that glow come from?

The idea

A black light gives out ultraviolet light, which you cannot see. Some materials soak up ultraviolet and give out visible light instead, so they glow. This is called fluorescence. Highlighter ink, some washing powders and some rocks fluoresce.

Ultraviolet carries more energy than visible light. That makes it useful, and also harmful:

  • it can kill germs, so hospitals use ultraviolet lamps to clean some equipment;
  • it fades colours in curtains and posters left in the sun;
  • it burns skin and can hurt eyes.

Some animals can see ultraviolet. Bees see patterns on flowers in ultraviolet that lead them to the nectar.

black light: ultravioletthe ink glows

Worked examples

1. Glowing ink. The highlighter ink takes in ultraviolet you cannot see and gives out green light you can.

2. A faded poster. A poster in a sunny window fades over a summer. Ultraviolet breaks down the colours in its ink.

3. Bees. A flower that looks plain yellow to us may have dark ultraviolet stripes that a bee can see.

Try it

  1. What does a black light give out?
  2. What is it called when something soaks up ultraviolet and glows?
  3. Name one useful thing ultraviolet can do.
  4. Name one harmful thing ultraviolet can do.
Answers
  1. Ultraviolet light.
  2. Fluorescence.
  3. Kill germs.
  4. Any one: burn skin, hurt eyes, fade colours.

Common mistake

"A black light gives out black light." It gives out ultraviolet, with a little violet that you see as a dim purple glow. Black is not a light.

Where it leads