The canon of light


Image credit: NASA/JPL-Caltech/STScI If it’s daylight look around you, at everything illuminated by the sun. If it’s during the night look out at the starlight, filtering down from so far away. If you’re indoors look at the light bulbs and monitor screens as they pour light out into the room. Now for a moment think about light, just light.

Whether it’s bright hot light from the sun or a cold white buzzing light from an incandescent light bulb or a blinking red light from the LED on your television, it’s all somehow the same. We recognise that it’s the same form of energy emitted, ricocheted, splashed and then absorbed. Some of that energy makes it into our own pupils allowing us to build up our visual picture of the world.

It is clear that the light that enters our pupils is only a tiny fraction of the light that surrounds us. However it is not only the quantity that is tiny but also the type of light that we can see. Our visual domain is just a sliver in the canon of light. Hidden behind names such as radio waves, microwaves, X-rays and gamma rays we find light that our eyes simply can’t see.

So all these are the same as light? Yes, in that red light is the same as blue light. They are both emissions of electromagnetic energy, but with different amounts of energy locked up inside. Being like a wave, the energy locked up is related to the wavelength of light. Shorter wavelengths have higher energies; blue light is more energetic than red light.

So what can we see? We can see light between wavelengths of 390 nanometres and 750 nanometres (a nanometre being a thousandth of a millionth of a metre), in terms of frequency that’s just short of an octave. Outside that range on the lower energies we have infrared, microwaves and radio waves whose wavelengths stretch to hundreds of metres long. Past blue towards the higher energies we have X-rays and gamma rays. The short wavelength and high energies of X-rays allow them to zoom through flesh, only stopping when they hit bones.

Each part of the entire spectrum finds it’s use in our highly technological world. Starting at low energies we find radio waves which carry our television and radio signals. Increasing in energy we next find microwaves which are at the heart of modern day communications, allowing GPS, radar, mobile phones and wireless networks to work; they even find their use heating up your cold food! Before reaching visible light we have infrared light, an important component in greenhouse heating, night-vision cameras and remote controls.

Beyond visible light we find ultraviolet, a light that can be dangerous to our skin but also finds its use in medical and industrial applications. With even higher energies we find X-rays, powerful enough to pierce flesh and diagnose our breaks and fractures. Climbing to higher energies still we have gamma rays, which have wavelengths as small, and smaller, than atoms. With these properties gamma rays can be used for medical applications such as imaging in PET scans and treating cancerous tissue.

From our small window of visible light we can now comprehend the vastness of light as it extends in both directions, past red and blue. And yet there is still much more to know about light that the eye can not see. As well as having a wavelength light can also vibrate in different directions. This polarisation finds its uses from very practical, such as making radar and satellite communications more efficient, to simply entertaining, by making 3D film and television possible.

Imagine your eye’s ability to see was stretched to encompass light from radio waves straight through to gamma rays and even polarisation. What would the world look like?

I imagine seeing a background glow, the leftover of the Big Bang, The Cosmic Microwave Background Radiation, a uniform glow of microwave light over thirteen billion years old. Layered over this would be stars as we normally see them with an abundance of new additions, such as GPS satellites weaving through the sky. Flashes of gamma rays would be seen arriving from the cosmos. In an urban environment the sky would be ablaze with television and radio signals. All around, pockets, bags and computers would be bright blinking beacons flashing in response to much brighter antennas in the distance.

Our little octave of light that we can see is a much more peaceful place to live than the one depicted above. But by stretching our understanding beyond this range we have gained access to the full canon of light and our world simply wouldn’t be the same without that understanding.