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What it is.
I’ve been trying a new (new to me) photographic technique recently. It is referred to as infrared photography. Most of the people who end up reading this post will probably know that what we see as white light is actually composed of a number of different colours. You can see those colours every time you see a rainbow.
When you see a rainbow the colours you see go from red at one side, to violet on the other. Beyond red is a part of the visible spectrum (of electromagnetic waves) that our eyes cannot see, called infrared (strongly heated metal, like the heating element of an electric stove produces lots of infrared). For the curious, beyond violet is ultra violet which can cause sunburn and eye damage if you go out into the sun unprotected. But to avoid misunderstanding, the infrared photography I am discussing is not thermal imaging, which is photography or videography of radiated infrared. What I am discussing is reflected infrared light.
It is the reflected infrared light that photographers are usually interested in, either trying to block it out completely or to make use of it to provide some very interesting photographs.
The visible spectrum, what you can see in a rainbow, runs from 400nm to 700nm approximately. If you want the gory details, see here. The part of the infra red spectrum that photographers are interested in runs from beyond 700nm to approximately 1400nm. This is sometimes referred to as near infra red.