Ken Tapping, 6th November, 2018
In the 18th century, Isaac Newton passed white light through a prism, splitting it into a rainbow of colours. This happened because the different wavelengths of light, which our eyes and brains interpret as colours, are bent by different amounts when passed through a prism. At the beginning of the 19th century, William Herschel repeated Newton's experiment, but he went a step further. He scanned along the spectrum of rainbow colours with a thermometer. The bulb of the thermometer was blackened with carbon powder so that it would absorb any light falling on it. He found, probably as expected, that the various colours all warmed the bulb, but then found something odd, when he held the thermometer beyond the red end of the spectrum, where nothing was visible, the thermometer showed a temperature increase. It was being warmed by some sort of invisible light. This light became known as "infrared" (below red, or "redder than red"). The experiment showed that what we call radiant heat is actually a longer-wavelength form of light. One of the most scientifically useful aspects of this infrared light is that it is emitted by bodies as cold as a degree or so above absolute zero (-273 °C). Everything in the universe is warmer than this. Even the fading glow of the Big Bang – the beginning of the universe, almost 14 billion years ago – is mainly infrared.
When we look at an object like the Moon with our eyes, or using binoculars or an optical telescope, we see it because it is lit up by the Sun. When the Moon is not Full, we see only part of the half of it that is lit by the Sun. On the other hand, if we look at the Moon using an infrared telescope, or with a radio telescope operating at centimetre or shorter wavelengths, which is essentially a long-wavelength infrared telescope, we see the whole lunar disc, because we are seeing energy radiated from the Moon itself. The Moon's average temperature is about –50 °C, which is warm enough to glow brightly in the infrared. The sunlit part glows brighter because the surface is a bit warmer. We can use these measurements as a long-distance thermometer to measure the temperature of astronomical bodies. We can follow how the temperature of the Moon's surface changes during the lunar "day" and we can measure the temperatures of asteroids, and other objects.
Have you ever noticed that when driving in fog, during the day or night, orange-tinted glasses often help you see things more clearly? This is because the water droplets in mist or fog scatter the light, making it glow and blocking our vision. The amount of scattering depends very strongly on the wavelength of the light. Blue light has half the wavelength of red light and is scattered about sixteen times more seriously. By blocking out the more seriously scattered blue light, we can often see better, depending on the size and concentration of the water droplets.
Space is filled with a fog of gas and dust. If we look low in the south on summer evenings, we are looking in the direction of the centre of our galaxy. However, thanks to the dense clouds of gas and dust in that direction, we can only see a small fraction of the way there. However, if we observe at infrared wavelengths, those clouds of dust and gas are almost transparent, and we can see all the way to our galaxy's centre. In the same way we can look into the birth clouds of new stars and planets, and see the material collapsing into discs and then into new "solar systems".
Infrared observations show us things we cannot see with visible light, and are now an important branch of astronomy. However, considering the wide range of conditions and processes taking place out there, even then we don't get the whole story; we need to observe at other wavelengths.
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