Excerpt: ‘A Brief History of Light,’ by Upulie Divisekera

image

Photo by jimflix! Reproduced under the Creative Commons Attribution 2.0 Generic License.

“What language does light speak? Vowels hang down from the pepper tree in their green and their gold.” – Charles Wright, Cryopexy

The pepper tree is an ornamental, invasive tree from South America, but it can sometimes be found lining the sides of Melbourne roads. Transplanted across the Pacific, it’s an oddly graceful tree: gnarled, twisted trunks made of splintery, rough grey bark; graceful, dark-green glossy and delicate leaves like fern fronds; fruit like bunches of peppercorns. Some use these pepperberries as substitutes for real peppercorns; the leaves are poisonous. The pepperberries take on hues of pink, green and yellow, literal “vowels” of light: absorbing some wavelengths, releasing others. The wavelengths the pigments reject, what we see as colour, form the language we use to describe the world. But the entire tree is transformed light: light captured, converted to wood, sugar, pepperberry, leaf; light made flesh, light made colour. The tree tells the story of the life of light, from the dawn of the universe to a petrified record of our star, the sun.

Light controls our world, gave rise to life and colour, and enables us to understand the universe.

Our days are governed by the apparent movement of the sun across the bowl of the sky; illuminated by it, warmed by it, our food depending upon the right balance of light and water. Our lives in turn are controlled by the rotation of our planet around our sun; we are inextricably linked to the interaction of sunlight with everything on Earth. Light controls our world, gave rise to life and colour, and enables us to understand the universe.

What is light?

That light is a strange substance has been known since we first perceived it. Its nature has been debated since antiquity. The Greek philosopher Empedocles thought fire shot out from the eyes, but could not account for our lack of night vision. The Renaissance scientist Robert Hooke imagined it as a “pulse,” similar to the waves and ripples you can see in a pond if you drop a pebble into it. Descartes found that light always travelled in a straight line. Isaac Newton thought that light was made of “corpuscles,” or particles. Particles of light. That would mean discrete entities, but we see sunbeams spreading like waves, soft edges around pale shadows in late morning. Newton studied the behaviour of light extensively; his great work Opticks taught us the first rules of light: that light slows down when passing through water and glass, and how light behaves with mirrors of different shapes. But most importantly, Newton was the first to split light. Passing a beam of sunlight through a prism, he found he had created an earthly rainbow. Passing this rainbow through another prism at the correct angle, he reunited the colours and made the light white again. From the one came many, a rainbow and a whole world hidden from our view. Here was the first part of the electromagnetic spectrum, the radiation which extends from very high-energy X-rays, through the seven colours of visible light, to low-energy infrared – that warping of your vision, that shimmering heat haze you see over a fire. All of this energy is of the same nature, related to each other.


This piece appears in full in The Lifted Brow #26. Get your copy now.

Upulie Divisekera is a molecular biologist. She founded the science outreach program Real Scientists, and communicates science through writing, performance and radio.