Wonders of the Universe

ALL THE COLOURS OF THE RAINBOW

The breathtaking Victoria Falls are one of the most famous and beautiful natural wonders on our planet. Fuelled by the mighty Zambezi River, the falls lie on the border between Zambia and Zimbabwe in southern Africa. The falls were named by David Livingstone in 1855, the first European to see them. He later wrote: ‘No one can imagine the beauty of the view from anything witnessed in England. It had never been seen before by European eyes; but scenes so lovely must have been gazed upon by angels in their flight.’ That’s about right from where I stood. There are few better places on Earth from which you can experience the visceral power of flowing water, but there is an ethereal feature of the falls that is just as enchanting and far more instructive for our purposes, because it holds the key to interpreting the Hubble Deep Field Image.

Hovering in the skies above the falls are magnificent rainbows, a permanent feature in the Zambian skies when the Sun shines through the mist. Rainbows are natural phenomena that have enchanted humans for thousands of years; to see one is to marvel at a simple but beautiful property of light and, as is often the case in nature, they are made more beautiful when you understand the science behind them.

Scientists have attempted to understand rainbows since the time of Aristotle, trying to explain how white light is apparently transformed into colour. Our old friend Ibn al-Haytham was one of the first to attempt to explain the physical basis of a rainbow in the tenth century. He described them as being produced by the ‘light from the Sun as it is reflected by a cloud before reaching the eye’. This isn’t too far from the truth. The basis of our modern understanding was delivered by Isaac Newton, who observed that white light is split into its component colours when passed through a glass prism. He correctly surmised that white light is made up of light of all colours, mixed together. The physics behind the production of a rainbow is essentially the same as that of the prism. Light from the Sun is a mixture of all colours, and water droplets in the sky act like tiny prisms, splitting up the sunlight again. But why the characteristic arc of the rainbow?

The first scientific explanation, which pre-dated Newton by several decades, was given by René Descartes in 1637. Water droplets in the air are essentially little spheres of water, so Descartes considered what happens to a single ray of light from the Sun as it enters a single water droplet. As the diagram opposite illustrates, the light ray from the Sun (S) enters the face of the droplet and is bent slightly. This is known as refraction; light gets deflected when it crosses a boundary between two different substances (point A), then when the light ray gets to the back surface of the raindrop, it is reflected back into the raindrop (point B), finally emerging out of the front again, where it gets bent a little more (point C). The light ray then travels from the raindrop to your eye (E).

The key point is that there is a maximum angle (D) through which light that enters the raindrop gets bounced back. Descartes calculated this angle for red light and found it to be forty-two degrees. For blue light, the angle is forty degrees. Colours between blue and red in the spectrum have maximum angles of reflection of between forty-two and forty degrees. No light gets bounced back with angles greater than this, and it turns out that most of the light gets reflected back at this special, maximum angle. So, here is the explanation for the rainbow. When you look up at a rainbow, imagine drawing a line between the Sun, which must be behind you, through your head and onto the ground in front of you. At an angle of forty-two degrees to this line, you’ll see the so-called rainbow, or Descartes’ ray of red light. At an angle of forty degrees to this line, you’ll see the Descartes’ ray of blue light, and all the colours of the rainbow in between. There is some light reflected back to your eye through shallower angles, which is why the sky is brighter below the arc than above it. You don’t see the colours below the arc because all the rays merge to form white light. On the picture on the previous page, you can see the sky brightening inside the rainbow over the Victoria Falls, and the relative darkness of the sky outside it.

So raindrops separate the white sunlight into a rainbow because each of the consituent colours gets reflected back to your eye at a slightly different maximum angle. But why the arc? In fact, rainbows are circular. Think of the imaginary line again between the Sun, your head and the ground. There isn’t just one place at which the angle between this line and the sky is forty-two degrees, there is a whole circle of points surrounding the line. The reason you can’t see a complete circle is that the horizon cuts it off, so you only see the arc. This is also why you tend to see rainbows in the early morning or late afternoon. As the Sun climbs in the sky, the line between the Sun and your head steepens and the rainbow, which is centred on this line, drops closer and closer to the horizon until at some point it will vanish below the horizon.

All the way back to Aristotle, scientists have been trying to understand rainbows and how white light is transformed to colour through this medium. The Victoria Falls are perhaps one of the most spectacular places on Earth to see rainbows; here, these features hover in the sky above the cascading waters whenever the Sun shines through the mist.

THE ELECTROMAGNETIC SPECTRUM

The electromagnetic spectrum is composed of a range of wavelengths from radio waves at the very longest end to gamma rays at the shortest. Our eyes are sensitive to a limited range in the middle which we know as visible light.

In fact, rainbows are circular. The reason you can’t see a complete circle is that the horizon cuts it off, so you only see the arc. This is also why you tend to see rainbows in the early morning or late afternoon.

WHAT MAKES A RAINBOW AN ARC?

Decartes’ theory was based on what happens to a single ray of light from the Sun as it enters a water droplet; he discovered that each colour that makes up this light is refracted, or bent, at slightly different angles to each other.

These colours hidden in white light are not only revealed in rainbows; wherever sunlight strikes an object the different colours are bounced around or absorbed in different ways. The sky is blue because the blue components of sunlight are more likely to be scattered off air molecules than the other colours. As the Sun drops towards the horizon, and the sunlight has to pass through more of the atmosphere, the chance of scattering rays of yellow and red light increases, turning the evening skies redder. Leaves and grass are green because they absorb blue and red light from the Sun, which they use in photosynthesis, but reflect back the green light.

But what is the difference between the colours that makes them behave so differently? The answer goes back to our understanding of light as an electromagnetic wave. Waves have a wavelength – which is the distance between two peaks (or troughs) of the wave. Blue light has a shorter wavelength than green light, which has a shorter wavelength than red light. Our eye has evolved to discern about ten million different colours, which is to say that it can differentiate between ten million subtle variations in the wavelength of electromagnetic waves. This simple idea is all you need to read the story of the Hubble Ultra Deep Field image