You can’t see the infrared light because it is just outside the range of the spectrum which is visible to the cones in our eyes. You almost certainly feel it and use it every day.

Objects may not be glowing, but if they are emitting heat then they are often emitting infrared light. On contact with your skin, this light is converted to heat. The footpath on a hot day is an infrared light light source.

Infrared light lies between the visible and microwave sections of the electromagnetic spectrum. Like each of the bands of the electromagnetic spectrum, infrared light has a range of wavelengths.

With far infrared that we are still within the realm of light with wavelengths we can easily conceive. The longest wavelengths of infrared light are about the size of a pin head, about a millimetre. Near infrared light is closest in wavelength to visible light, with wavelengths smaller than our familiar scales.

Seeing Infrared

The primary source of infrared radiation is heat or thermal radiation. If an object is warm, it is radiating infrared. Hot objects emit visible light - flames and red hot burners. If an object is not quite hot enough to radiate visible light, then it will emit infrared energy.

The warmer the object, the more infrared radiation it emits.

The heat that you feel from sunlight and a hot pavement is infrared.

The picture above is an attempt by the author to make a normal picture look hot. Infrared cameras can do the real thing - they can show you heat.

To make infrared pictures like the cat to the left, special cameras and film are used. These are sensitive to the temperature variation in the object being photographed.

The camera then assigns colours from the visible light spectrum to the temperature gradients. These colours are false, but they serve to create an image that our eyes can interpret.

Infrared Cat images courtesy SE-IR Corporation, Goleta, California, USA.

The cat to the right is photographed in terms of its infrared radiation. Some may say this is the only time a cat is photographed being warm!

The warmest areas show up as orange, varying to the coldest, such as the tip of the nose, showing up blue to white.

The information revealed by infrared is different to that revealed by photographing in the visible light range. We have lost fur colour and gained fur temperature.

Television remotes are infrared

The signal from your television remote control is infrared. It travels from the remote to the TV, where the signal is detected and interpreted. You will have already conducted experiments using the remote without realising it. When you don't get a reaction from the TV, you change what you are doing, trying to improve the direction or remove blockages. When the remote has a new battery, it seems to work fine no matter what direction you point it.

1. If light always travels in straight lines, how can the signal from the remote work when you are pointing away from the television?

Hypothesis: If an object is transparent to visible light, it is transparent to all
electromagnetic radiation, including infrared.

Corollary: If an object is opaque to visible light, it is opaque to all
electromagnetic radiation, including infrared.