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What blocks ultraviolet

Radiation and the atom · level 2, lesson 5 of 6

A question to start

You cannot see ultraviolet. So how can you test whether a T-shirt, a window or sunscreen blocks it?

The idea

You can use things that change colour in ultraviolet. Some beads go from white to bright colours in sunshine, and fade back to white indoors. Put something between the beads and the Sun: if the beads stay white, it blocks ultraviolet.

What you might find:

  • Thick clothes, card, wood and walls block it.
  • Sunscreen spread on a clear plastic sheet blocks a lot of it.
  • Window glass blocks much of it, but not all, so you can still slowly tan or fade through a window.
  • Thin cloud and shallow water let a lot of it through.
open sununder a T-shirtunder sunscreen

Worked examples

1. A T-shirt test. Beads under a thick T-shirt stay white. Under a thin, loose one, they turn a little pale purple: some ultraviolet gets through.

2. A window. Beads behind a closed window change colour less than beads outside.

3. In the shade. Beads in the shade of a tree still change a little. Ultraviolet bounces off the sky and the ground, and reaches the shade too.

Try it

  1. How can you tell where ultraviolet is, when you cannot see it?
  2. Does a wooden fence block ultraviolet?
  3. Does window glass block all ultraviolet?
  4. Why do the beads change a little even in shade?
Answers
  1. Use something that changes colour in ultraviolet, like UV beads.
  2. Yes.
  3. No, it blocks much of it, but not all.
  4. Ultraviolet scatters off the sky and reflects off the ground into the shade.

Common mistake

"Shade blocks all the ultraviolet." Shade helps a lot, but scattered ultraviolet still reaches you. On a bright day, wear a hat and sunscreen in the shade too.

Where it leads