Waves

Electromagnetic Waves - Infra-red.

Infra-red waves were discovered by Sir William Herschel in 1800.
This was the first discovery of electromagnetic waves
outside the range of the visible spectrum.

Infra-red waves (often called infra-red radiation or thermal radiation)
are easily absorbed by materials.
The energy of the wave causes the material to get hot.
We usually think of infra-red radiation as heat.

Ordinary ovens, grills and toasters use infra-red radiation to cook food
(ovens may also cook by convection).
Intense infra-red radiation will damage or kill living cells
(such as skin cells) by burning them.

Infra-red waves can transmit information through the air
to operate TV's and VCR's by remote control.
Information can also be sent through optical fibres.

Infra-red waves are used for thermal imaging.
In a thermal imaging device,
a detector receives infra-red waves and produces an image
where different colours show the intensity of the waves in different places.
This gives you a 'heat picture' where hotter objects are orange / red
and colder objects are blue / green.
Thermal imaging is used by fire fighters
to see where the hottest part of the fire is.

A device called a PIR (passive infra-red) is used for security.
When a warm object such as a human or animal approaches a house
the PIR can detect it against the colder background
and send a signal to switch on an outside light.

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