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Infrared (IR)

Ignoring the fact that some of us have to wear glasses or contact lenses (I occasionally wear glasses); the majority of us have, thankfully, a good eye sight. Vision is captured by our eyes and the respective retinas convert the sight into electrical signals. The optic nerve transmits the signals to the brain, which interprets the message and informs us of what we are seeing. That is a simplistic way of describing our biological visual system, though the question is based, in physics, as to how objects are visible?

Our eyes are capable of seeing the visible spectrum (also known as visible light or white light, yet simply, light) and it is a small part of the electromagnetic spectrum (EM spectrum). The main source of the EM spectrum is from the sun and on a day without clouds, it is brightest at noon. (The speed of the EM spectrum, in a vacuum, is 186,282 miles per second). The visible spectrum has different colours and below is a table showing them; however if you are expecting to see indigo, it is not shown. In Sir Isaac Newton's book, Opticks, he used the Latin word spectrum (in English, appearance) and divided the book into seven categories with each related to the colours. Indigo is between violet and blue. As it is virtually impossible to spot indigo current scientists removed it from the spectrum. Beside the colours you will see the respective frequencies and the measurement in nanometres (nm). The point of showing the frequencies is due to our eyesight, as we can see anything that is within roughly 390-700nm:

Violet	380-450nm
Blue	450-495nm
Green	495-570nm
Yellow	575-585nm
Orange	590-620nm
Red	625-740nm

You will notice that violet appears to start at below 390nm and that red ends above 700nm. Things do not always start and finish at the same point and to correlate this, you can see the separate colours by looking at the next rainbow.

The illustration shows how white light passing through a prism can create the respective colours. Notice how red is refracted less so that violet and that is based on their frequencies:

By looking at a rainbow you are seeing all the separate colours; however suppose you are looking at an apple, what makes it look red? As mentioned earlier, visible spectrum is also known as white light and the word white is used as no colours are shown. When the white light streams onto an object there are three things that can happen: 1) Transmitted, 2) Absorbed and 3) Reflected. Dependent on the result, two things can happen: 1) Transparent and 2) Opaque. The table shows how this works:

Transparent	The visual spectrum's frequencies do not match the object's frequencies and they will be "transmitted" through the object.
Opaque	Parts of the visual spectrum's frequency do match and they will be "absorbed" in the object. The remaining wavelengths will be "reflected" to our eyes and our brain gives us the object's colour.

Before discussing IR, remember that due to the reflection of the visible spectrum from the object, we know its colour. It is not that the object itself has a colour. To clarify, consider that on a very dark night you would be unable to tell the colour of the apple, yet in daylight you will know.

Previously I mentioned that the frequency of the visible spectrum is from circa 390-700nm and that it is just a small part of the EM spectrum. The table below shows that spectrum:

Gamma Rays	From lower to 10 picometres (pm)
X-rays	0.01-10nm
Ultraviolet	10-400nm
Visible Light	390-700nm
Infrared	700nm-1 millimetre (mm)
Microwaves	1mm-1 metre (m)
Radio Waves	1m-100 Megametres (Mm)

On a sunny day in 1800 Sir William Herschel was testing the effect of light and heat. The darkroom was set-up to allow a small stream of bright light to enter a window. The light passed through a prism, which in turn created a rainbow. Arranged thermometers captured the separate colour, allowing him to observe and note the readings. He found that with violet, and so too the other colours, the temperature shown on the thermometer increased slower than that of red. This information triggered him to search further and on another day he arranged for the thermometers to register anything that he cannot see, namely above red. In essence he uncovered IR.

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