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Light you cannot see

Radiation and the atom · level 2, lesson 1 of 6

A question to start

Hold your hand near a warm radiator with your eyes shut. You can feel warmth coming from it, but you cannot see any glow. What is reaching your hand?

The idea

Sunlight is made of the colours of the rainbow, from red to violet. But there is more to it that our eyes cannot see.

  • Just past red is infrared. We feel it as warmth. Warm things, like a radiator or your body, give it out.
  • Just past violet is ultraviolet, often called UV. It carries more energy than violet light. It is what causes sunburn.

In 1800, William Herschel spread sunlight out with a prism and put thermometers in each colour. A thermometer just past the red end, where there was no colour at all, got the warmest. He had found infrared.

ultravioletinfraredcolours we seecauses sunburnfelt as warmth

Worked examples

1. A warm cup. A cup of hot cocoa gives out infrared. Hold your hands round it and you feel it, without seeing any glow.

2. A TV remote. It sends an infrared beam. Look at it through a phone camera while you press a button: many cameras show a flicker your eyes cannot see.

3. A cloudy day. Clouds block a lot of visible light, but much of the ultraviolet still gets through. You can get sunburnt on a cloudy day.

Try it

  1. What is the light just past red called?
  2. What is the light just past violet called?
  3. How did Herschel find infrared?
  4. Can you get sunburnt on a cloudy day?
Answers
  1. Infrared.
  2. Ultraviolet, or UV.
  3. He put thermometers in the colours of sunlight, and the one past the red end got warmest.
  4. Yes, much of the ultraviolet still gets through clouds.

Common mistake

"If you cannot see it, it is not light." Infrared and ultraviolet are light too, just colours our eyes cannot see.

Where it leads